[Clang] Emit DW_TAG_template_alias for template aliases (#87623)

Fix issue https://github.com/llvm/llvm-project/issues/54624

Add front end flags -gtemplate-alias (also a cc1 flag) and -gno-template-alias
to enable/disable usage of the feature. It's enabled by default in the front
end for SCE debugger tuning only.

GCC emits DW_TAG_typedef for template aliases (as does Clang with this feature
disabled), and GDB and LLDB appear not to support DW_TAG_template_alias.

The -Xclang option -gsimple-template-names=mangled is treated the same as
=full, which is not a regression from current behaviour for template
aliases.

The current implementation omits defaulted arguments and as a consequence
also omits empty parameter packs that come after defaulted arguments. Again,
this isn't a regression as the DW_TAG_typedef name doesn't contain defaulted
arguments.

LLVM support added in https://github.com/llvm/llvm-project/pull/88943

Added template-alias.cpp - Check the metadata construction & interaction with
      -gsimple-template-names.
Added variadic-template-alias.cpp - Check template parameter packs work.
Added defaulted-template-alias.cpp - Check defaulted args (don't) work.
Modified debug-options.c - Check Clang generates correct cc1 flags.

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 1f1f544..5531e93 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1556,6 +1556,9 @@ void CompilerInvocationBase::GenerateCodeGenArgs(const CodeGenOptions &Opts,
                llvm::DICompileUnit::DebugNameTableKind::Default))
     GenerateArg(Consumer, OPT_gpubnames);
 
+  if (Opts.DebugTemplateAlias)
+    GenerateArg(Consumer, OPT_gtemplate_alias);
+
   auto TNK = Opts.getDebugSimpleTemplateNames();
   if (TNK != llvm::codegenoptions::DebugTemplateNamesKind::Full) {
     if (TNK == llvm::codegenoptions::DebugTemplateNamesKind::Simple)
@@ -1827,6 +1830,8 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
   Opts.BinutilsVersion =
       std::string(Args.getLastArgValue(OPT_fbinutils_version_EQ));
 
+  Opts.DebugTemplateAlias = Args.hasArg(OPT_gtemplate_alias);
+
   Opts.DebugNameTable = static_cast<unsigned>(
       Args.hasArg(OPT_ggnu_pubnames)
           ? llvm::DICompileUnit::DebugNameTableKind::GNU