[clang] [Clang] [AST] Fix placeholder return type name mangling for MSVC 1920+ / VS2019+ (PR #102848)

[Max Winkler via cfe-commits]

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@@ -2494,6 +2506,58 @@ void MicrosoftCXXNameMangler::mangleAddressSpaceType(QualType T,
   mangleArtificialTagType(TagTypeKind::Struct, ASMangling, {"__clang"});
 }
 
+void MicrosoftCXXNameMangler::mangleAutoReturnType(QualType T,
+                                                   SourceRange Range,
+                                                   QualifierMangleMode QMM) {
+  assert(getASTContext().getLangOpts().isCompatibleWithMSVC(
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MaxEW707 wrote:

The assert is here because the VS2017 path still takes the incorrect manglings path. I want to ensure any potential changing of the code doesn't call this function when mangling for VS2017 or earlier.

That happens here, https://github.com/llvm/llvm-project/pull/102848/files#diff-f5b0f26263c3f9e2967ea9e3911d88ce9c4b3e990cbbab10627429435305bb96R2997, where we check if we are VS2019 otherwise do the incorrect mangling.

https://github.com/llvm/llvm-project/pull/102848