Rollup merge of #104511 - dpaoliello:privateglobalworkaround, r=michaelwoerister

Mark functions created for `raw-dylib` on x86 with DllImport storage class

Fix for #104453

## Issue Details
On x86 Windows, LLVM uses 'L' as the prefix for any private global symbols (`PrivateGlobalPrefix`), so when the `raw-dylib` feature creates an undecorated function symbol that begins with an 'L' LLVM misinterprets that as a private global symbol that it created and so fails the compilation at a later stage since such a symbol must have a definition.

## Fix Details
Mark the function we are creating for `raw-dylib` with `DllImport` storage class (this was already being done for MSVC at a later point for `callee::get_fn` but not for GNU (due to "backwards compatibility")): this will cause LLVM to prefix the name with `__imp_` and so it won't mistake it for a private global symbol.

src/test/run-make/raw-dylib-import-name-type/driver.rs

@@ -3,6 +3,7 @@
 
 #[link(name = "extern", kind = "raw-dylib", import_name_type = "undecorated")]
 extern "C" {
+    fn LooksLikeAPrivateGlobal(i: i32);
     fn cdecl_fn_undecorated(i: i32);
     #[link_name = "cdecl_fn_undecorated2"]
     fn cdecl_fn_undecorated_renamed(i: i32);
@@ -84,6 +85,13 @@ extern {
 
 pub fn main() {
     unsafe {
+        // Regression test for #104453
+        // On x86 LLVM uses 'L' as the prefix for private globals (PrivateGlobalPrefix), which
+        // causes it to believe that undecorated functions starting with 'L' are actually temporary
+        // symbols that it generated, which causes a later check to fail as the symbols we are
+        // creating don't have definitions (whereas all temporary symbols do).
+        LooksLikeAPrivateGlobal(13);
+
         cdecl_fn_undecorated(1);
         cdecl_fn_undecorated_renamed(10);
         cdecl_fn_noprefix(2);

src/test/run-make/raw-dylib-import-name-type/extern.c

@@ -1,6 +1,11 @@
 #include <stdio.h>
 #include <stdint.h>
 
+void _cdecl LooksLikeAPrivateGlobal(int i) {
+    printf("LooksLikeAPrivateGlobal(%d)\n", i);
+    fflush(stdout);
+}
+
 void _cdecl cdecl_fn_undecorated(int i) {
     printf("cdecl_fn_undecorated(%d)\n", i);
     fflush(stdout);

src/test/run-make/raw-dylib-import-name-type/extern.gnu.def

@@ -1,5 +1,6 @@
 LIBRARY extern
 EXPORTS
+    LooksLikeAPrivateGlobal
     cdecl_fn_undecorated
     cdecl_fn_undecorated2
     cdecl_fn_noprefix

src/test/run-make/raw-dylib-import-name-type/extern.msvc.def

@@ -1,5 +1,6 @@
 LIBRARY extern
 EXPORTS
+    LooksLikeAPrivateGlobal
     cdecl_fn_undecorated
     cdecl_fn_undecorated2
     cdecl_fn_noprefix

src/test/run-make/raw-dylib-import-name-type/output.txt

@@ -1,3 +1,4 @@
+LooksLikeAPrivateGlobal(13)
 cdecl_fn_undecorated(1)
 cdecl_fn_undecorated2(10)
 cdecl_fn_noprefix(2)