rustc: Continue to tweak "std internal symbols"

In investigating [an issue][1] with `panic_implementation` defined in an
executable that's optimized I once again got to rethinking a bit about the
`rustc_std_internal_symbol` attribute as well as weak lang items. We've sort of
been non-stop tweaking these items ever since their inception, and this
continues to the trend.

The crux of the bug was that in the reachability we have a [different branch][2]
for non-library builds which meant that weak lang items (and std internal
symbols) weren't considered reachable, causing them to get eliminiated by
ThinLTO passes. The fix was to basically tweak that branch to consider these
symbols to ensure that they're propagated all the way to the linker.

Along the way I've attempted to erode the distinction between std internal
symbols and weak lang items by having weak lang items automatically configure
fields of `CodegenFnAttrs`. That way most code no longer even considers weak
lang items and they're simply considered normal functions with attributes about
the ABI.

In the end this fixes the final comment of #51342

[1]: https://github.com/rust-lang/rust/issues/51342#issuecomment-414368019
[2]: 35bf1ae257/src/librustc/middle/reachable.rs (L225-L238)

src/test/run-make/wasm-symbols-not-imported/Makefile

@@ -0,0 +1,16 @@
+-include ../../run-make-fulldeps/tools.mk
+
+ifeq ($(TARGET),wasm32-unknown-unknown)
+all:
+	$(RUSTC) foo.rs --target wasm32-unknown-unknown
+	$(NODE) verify-no-imports.js $(TMPDIR)/foo.wasm
+	$(RUSTC) foo.rs --target wasm32-unknown-unknown -C lto
+	$(NODE) verify-no-imports.js $(TMPDIR)/foo.wasm
+	$(RUSTC) foo.rs --target wasm32-unknown-unknown -O
+	$(NODE) verify-no-imports.js $(TMPDIR)/foo.wasm
+	$(RUSTC) foo.rs --target wasm32-unknown-unknown -O -C lto
+	$(NODE) verify-no-imports.js $(TMPDIR)/foo.wasm
+else
+all:
+endif
+

src/test/run-make/wasm-symbols-not-imported/foo.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type = "cdylib"]
+
+#![feature(panic_implementation)]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[no_mangle]
+pub extern fn foo() {
+    panic!()
+}
+
+#[panic_implementation]
+fn panic(_info: &PanicInfo) -> ! {
+    loop {}
+}

src/test/run-make/wasm-symbols-not-imported/verify-no-imports.js

@@ -0,0 +1,20 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+const fs = require('fs');
+const process = require('process');
+const assert = require('assert');
+const buffer = fs.readFileSync(process.argv[2]);
+
+let m = new WebAssembly.Module(buffer);
+let list = WebAssembly.Module.imports(m);
+console.log('imports', list);
+if (list.length !== 0)
+  throw new Error("there are some imports");

src/test/ui/panic-handler/panic-handler-std.stderr

@@ -8,6 +8,12 @@ LL | | }
    |
    = note: first defined in crate `std`.
 
-error: aborting due to previous error
+error: argument should be `&PanicInfo`
+  --> $DIR/panic-handler-std.rs:18:16
+   |
+LL | fn panic(info: PanicInfo) -> ! {
+   |                ^^^^^^^^^
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0152`.