[Object][COFF] Avoid underscore prefix for forwarding exports (#135433)

Fixes #132411.

lld/test/COFF/export-alias.test

@@ -0,0 +1,16 @@
+; REQUIRES: x86
+
+; RUN: echo > %t.s
+; RUN: llvm-mc -filetype=obj -triple=i686-windows %t.s -o %t.obj
+; RUN: lld-link -machine:x86 -dll -noentry -safeseh:no -out:%t.dll -def:%s %t.obj
+; RUN: llvm-readobj --coff-exports %t.dll | FileCheck %s
+
+; CHECK:      Export {
+; CHECK-NEXT:   Ordinal: 1
+; CHECK-NEXT:   Name: func
+; CHECK-NEXT:   ForwardedTo: other.dll.func
+; CHECK-NEXT: }
+
+LIBRARY test.dll
+EXPORTS
+        func = other.dll.func

llvm/lib/Object/COFFModuleDefinition.cpp

@@ -238,7 +238,10 @@ private:
     }
 
     if (AddUnderscores) {
-      if (!isDecorated(E.Name, MingwDef))
+      // Don't add underscore if the name is already mangled or if it's a
+      // forward target.
+      if (!isDecorated(E.Name, MingwDef) &&
+          (E.ExtName.empty() || !StringRef(E.Name).contains(".")))
         E.Name = (std::string("_").append(E.Name));
       if (!E.ExtName.empty() && !isDecorated(E.ExtName, MingwDef))
         E.ExtName = (std::string("_").append(E.ExtName));

llvm/test/tools/llvm-lib/underscore.test

@@ -0,0 +1,23 @@
+; RUN: llvm-lib -machine:x86 -out:%t.lib -def:%s
+; RUN: llvm-readobj %t.lib | FileCheck %s
+
+; CHECK:      File: test.dll
+; CHECK-NEXT: Format: COFF-import-file-i386
+; CHECK-NEXT: Type: code
+; CHECK-NEXT: Name type: noprefix
+; CHECK-NEXT: Export name: func
+; CHECK-NEXT: Symbol: __imp__func
+; CHECK-NEXT: Symbol: _func
+; CHECK-EMPTY:
+; CHECK-NEXT: File: test.dll
+; CHECK-NEXT: Format: COFF-import-file-i386
+; CHECK-NEXT: Type: code
+; CHECK-NEXT: Name type: noprefix
+; CHECK-NEXT: Export name: test.func
+; CHECK-NEXT: Symbol: __imp__test.func
+; CHECK-NEXT: Symbol: _test.func
+
+LIBRARY test.dll
+EXPORTS
+        func
+        test.func