Rollup merge of #53290 - whitequark:fix-35741, r=nagisa

Make LLVM emit assembly comments with -Z asm-comments

Fixes #35741, and makes `-Z asm-comments` actually do something useful.

Before:
```
	.section	.text.main,"ax",@progbits
	.globl	main
	.p2align	4, 0x90
	.type	main,@function
main:
	.cfi_startproc
	pushq	%rax
	.cfi_def_cfa_offset 16
	movslq	%edi, %rax
	leaq	_ZN1t4main17he95a7d4f1843730eE(%rip), %rdi
	movq	%rsi, (%rsp)
	movq	%rax, %rsi
	movq	(%rsp), %rdx
	callq	_ZN3std2rt10lang_start17h3121da83b2bc3697E
	movl	%eax, %ecx
	movl	%ecx, %eax
	popq	%rcx
	.cfi_def_cfa_offset 8
	retq
.Lfunc_end8:
	.size	main, .Lfunc_end8-main
	.cfi_endproc
```

After:
```
	.section	.text.main,"ax",@progbits
	.globl	main                    # -- Begin function main
	.p2align	4, 0x90
	.type	main,@function
main:                                   # @main
	.cfi_startproc
# %bb.0:
	pushq	%rax
	.cfi_def_cfa_offset 16
	movslq	%edi, %rax
	leaq	_ZN1t4main17he95a7d4f1843730eE(%rip), %rdi
	movq	%rsi, (%rsp)            # 8-byte Spill
	movq	%rax, %rsi
	movq	(%rsp), %rdx            # 8-byte Reload
	callq	_ZN3std2rt10lang_start17h3121da83b2bc3697E
	movl	%eax, %ecx
	movl	%ecx, %eax
	popq	%rcx
	.cfi_def_cfa_offset 8
	retq
.Lfunc_end8:
	.size	main, .Lfunc_end8-main
	.cfi_endproc
                                        # -- End function
```

src/librustc_codegen_llvm/back/write.rs

@@ -180,6 +180,8 @@ pub fn target_machine_factory(sess: &Session, find_features: bool)
     let is_pie_binary = !find_features && is_pie_binary(sess);
     let trap_unreachable = sess.target.target.options.trap_unreachable;
 
+    let asm_comments = sess.asm_comments();
+
     Arc::new(move || {
         let tm = unsafe {
             llvm::LLVMRustCreateTargetMachine(
@@ -193,6 +195,7 @@ pub fn target_machine_factory(sess: &Session, find_features: bool)
                 fdata_sections,
                 trap_unreachable,
                 singlethread,
+                asm_comments,
             )
         };
 

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -1455,7 +1455,8 @@ pub fn LLVMRustCreateTargetMachine(Triple: *const c_char,
                                        FunctionSections: bool,
                                        DataSections: bool,
                                        TrapUnreachable: bool,
-                                       Singlethread: bool)
+                                       Singlethread: bool,
+                                       AsmComments: bool)
                                        -> Option<&'static mut TargetMachine>;
     pub fn LLVMRustDisposeTargetMachine(T: &'static mut TargetMachine);
     pub fn LLVMRustAddAnalysisPasses(T: &'a TargetMachine, PM: &PassManager<'a>, M: &'a Module);

src/rustllvm/PassWrapper.cpp

@@ -366,7 +366,8 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
     bool PositionIndependentExecutable, bool FunctionSections,
     bool DataSections,
     bool TrapUnreachable,
-    bool Singlethread) {
+    bool Singlethread,
+    bool AsmComments) {
 
   auto OptLevel = fromRust(RustOptLevel);
   auto RM = fromRust(RustReloc);
@@ -393,6 +394,8 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
   }
   Options.DataSections = DataSections;
   Options.FunctionSections = FunctionSections;
+  Options.MCOptions.AsmVerbose = AsmComments;
+  Options.MCOptions.PreserveAsmComments = AsmComments;
 
   if (TrapUnreachable) {
     // Tell LLVM to codegen `unreachable` into an explicit trap instruction.