[lld][ELF] --warn-backrefs: Fix symbol name demangling

[nullptr.cpp]

Summary

In the warning of --warn-backrefs, symbol names should be:

• demangled with --demangle
• mangled with --no-demangle

[MaskRay]

I have noticed this inconsistency but have intended to not make a change because I actually find working with the original names easier...
When fixing internal projects for the backward reference problems, the original names allow me to use readelf -Ws something | grep ... more efficiently.

(Also I don't think new test files are needed. Reusing the existing should be fine.)

[nullptr.cpp]

Use the existing test

lld/ELF/Symbols.cpp

for (auto &it : backwardReferences) {
     // with --warn-backrefs-exclude=. to may look like *.o or *.a(*.o).
     bool exclude = false;
     for (const llvm::GlobPattern &pat : config->warnBackrefsExclude)
       if (pat.match(to)) {
         exclude = true;
         break;
       }
     if (!exclude)
-      warn("backward reference detected: " + sym.getName() + " in " +
+      warn("backward reference detected: " + toString(sym) + " in " +
            toString(it.second.first) + " refers to " + to);
   }
 }
 
 static uint8_t getMinVisibility(uint8_t va, uint8_t vb) {
   if (va == STV_DEFAULT)
     return vb;
   if (vb == STV_DEFAULT)

lld/test/ELF/lto/warn-backrefs.ll

 ; REQUIRES: x86
 ;; Test that the referenced filename is correct (not <internal> or lto.tmp).
 
 ; RUN: split-file %s %t
 ; RUN: llvm-as %t/a.ll -o %ta.o
 ; RUN: llvm-as %t/b.ll -o %tb.o
-; RUN: ld.lld --warn-backrefs --start-lib %tb.o --end-lib %ta.o -o /dev/null 2>&1 | FileCheck %s
+; RUN: ld.lld --demangle --warn-backrefs --start-lib %tb.o --end-lib %ta.o -o /dev/null 2>&1 | FileCheck %s -check-prefix=CHECK-DEMANGLE
+; RUN: ld.lld --no-demangle --warn-backrefs --start-lib %tb.o --end-lib %ta.o -o /dev/null 2>&1 | FileCheck %s -check-prefix=CHECK-NO-DEMANGLE
 
-; CHECK: warning: backward reference detected: f in {{.*}}a.o refers to {{.*}}b.o
+; CHECK-DEMANGLE: warning: backward reference detected: f() in {{.*}}a.o refers to {{.*}}b.o
+; CHECK-NO-DEMANGLE:: warning: backward reference detected: _Z1fv in {{.*}}a.o refers to {{.*}}b.o
 
 ;--- a.ll
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
-declare void @f()
+declare void @_Z1fv()
 
 define void @_start() {
 entry:
-  call void () @f()
+  call void () @_Z1fv()
   ret void
 }
 
 ;--- b.ll
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
-define void @f() {
+define void @_Z1fv() {
 entry:
   ret void
 }