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.
