Name the captured upvars for closures/generators in debuginfo

Previously, debuggers print closures as something like
```
y::main::closure-0 (0x7fffffffdd34)
```
The pointer actually references to an upvar. It is not
very obvious, especially for beginners.

It's because upvars don't have names before, as they
are packed into a tuple. This commit names the upvars,
so we can expect to see something like
```
y::main::closure-0 {_captured_ref__b: 0x[...]}
```

src/test/debuginfo/captured-fields.rs

@@ -0,0 +1,87 @@
+// compile-flags:-g
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command:run
+// gdb-command:print test
+// gdbr-check:$1 = captured_fields::main::{closure#0} {_captured_ref__my_ref__my_field1: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$2 = captured_fields::main::{closure#1} {_captured_ref__my_ref__my_field2: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$3 = captured_fields::main::{closure#2} {_captured_ref__my_ref: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$4 = captured_fields::main::{closure#3} {_captured_val__my_ref: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$5 = captured_fields::main::{closure#4} {_captured_val__my_var: captured_fields::MyStruct {my_field1: 11, my_field2: 22}}
+// gdb-command:continue
+
+// === LLDB TESTS ==================================================================================
+
+// lldb-command:run
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#0}) $0 = { _captured_ref__my_ref__my_field1 = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#1}) $1 = { _captured_ref__my_ref__my_field2 = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#2}) $2 = { _captured_ref__my_ref = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#3}) $3 = { _captured_val__my_ref = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#4}) $4 = { _captured_val__my_var = { my_field1 = 11 my_field2 = 22 } }
+// lldb-command:continue
+
+#![feature(capture_disjoint_fields)]
+#![allow(unused)]
+
+struct MyStruct {
+    my_field1: u32,
+    my_field2: u32,
+}
+
+fn main() {
+    let mut my_var = MyStruct {
+        my_field1: 11,
+        my_field2: 22,
+    };
+    let my_ref = &mut my_var;
+
+    let test = || {
+        let a = &mut my_ref.my_field1;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = &my_ref.my_field2;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = &my_ref;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = my_ref;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = my_var;
+    };
+
+    _zzz(); // #break
+}
+
+fn _zzz() {}

src/test/debuginfo/generator-objects.rs

@@ -11,16 +11,16 @@
 
 // gdb-command:run
 // gdb-command:print b
-// gdb-check:$1 = generator_objects::main::{generator#0}::Unresumed(0x[...])
+// gdb-check:$1 = generator_objects::main::{generator#0}::Unresumed{_captured_ref__a: 0x[...]}
 // gdb-command:continue
 // gdb-command:print b
-// gdb-check:$2 = generator_objects::main::{generator#0}::Suspend0{c: 6, d: 7, __0: 0x[...]}
+// gdb-check:$2 = generator_objects::main::{generator#0}::Suspend0{c: 6, d: 7, _captured_ref__a: 0x[...]}
 // gdb-command:continue
 // gdb-command:print b
-// gdb-check:$3 = generator_objects::main::{generator#0}::Suspend1{c: 7, d: 8, __0: 0x[...]}
+// gdb-check:$3 = generator_objects::main::{generator#0}::Suspend1{c: 7, d: 8, _captured_ref__a: 0x[...]}
 // gdb-command:continue
 // gdb-command:print b
-// gdb-check:$4 = generator_objects::main::{generator#0}::Returned(0x[...])
+// gdb-check:$4 = generator_objects::main::{generator#0}::Returned{_captured_ref__a: 0x[...]}
 
 // === LLDB TESTS ==================================================================================
 

src/test/debuginfo/issue-57822.rs

@@ -11,17 +11,17 @@
 // gdb-command:run
 
 // gdb-command:print g
-// gdb-check:$1 = issue_57822::main::{closure#1} (issue_57822::main::{closure#0} (1))
+// gdb-check:$1 = issue_57822::main::{closure#1} {_captured_val__f: issue_57822::main::{closure#0} {_captured_val__x: 1}}
 
 // gdb-command:print b
-// gdb-check:$2 = issue_57822::main::{generator#3}::Unresumed(issue_57822::main::{generator#2}::Unresumed(2))
+// gdb-check:$2 = issue_57822::main::{generator#3}::Unresumed{_captured_val__a: issue_57822::main::{generator#2}::Unresumed{_captured_val__y: 2}}
 
 // === LLDB TESTS ==================================================================================
 
 // lldb-command:run
 
 // lldb-command:print g
-// lldbg-check:(issue_57822::main::{closure#1}) $0 = { 0 = { 0 = 1 } }
+// lldbg-check:(issue_57822::main::{closure#1}) $0 = { _captured_val__f = { _captured_val__x = 1 } }
 
 // lldb-command:print b
 // lldbg-check:(issue_57822::main::{generator#3}) $1 =