clang: alias -static-{libstdc++,libgcc} for LLVM variants

[martell]

Summary

Alias -static-compiler-rt and static-libc++ to their gcc counterparts.
Currently invoking -static-libgcc or -static-libstdc++ will ensure only use of static libs.
In future if we want to do more accurate handling with better behaviour with the linker we can remove the alias and pass a custom flag.
For now this would be a great alias for sanity.

[rsmith]

It would be nice to have a flag here that's agnostic to the value supplied to -rtlib or -stdlib. Perhaps -static-rtlib (for either libgcc or compiler-rt) and -static-stdlib (for either libc++ or libstdc++). (Yes, it's bad that we use stdlib to mean "the C++ standard library", but that is the status quo.)

[MaskRay]

In D37726#1264233, @rsmith wrote:

It would be nice to have a flag here that's agnostic to the value supplied to -rtlib or -stdlib. Perhaps -static-rtlib (for either libgcc or compiler-rt) and -static-stdlib (for either libc++ or libstdc++). (Yes, it's bad that we use stdlib to mean "the C++ standard library", but that is the status quo.)

rC113891 introduced -stdlib... yes it would be nice if it were named -c++stdlib or -cxxstdlib... (I'm not good at naming)

We don't need to be 100% compatible with gcc here.. --push-state -Bstatic -lc++/-lstdc++ --pop-state can also be used. --pop-state works with ld.bfd/gold/lld

[martell]

Dropping in favour of D53238

include/clang/Driver/Options.td

 def segs__read__write__addr : Separate<["-"], "segs_read_write_addr">;
 def segs__read__ : Joined<["-"], "segs_read_">;
 def shared_libgcc : Flag<["-"], "shared-libgcc">;
 def shared : Flag<["-", "--"], "shared">;
 def single__module : Flag<["-"], "single_module">;
 def specs_EQ : Joined<["-", "--"], "specs=">;
 def specs : Separate<["-", "--"], "specs">, Flags<[Unsupported]>;
 def static_libgcc : Flag<["-"], "static-libgcc">;
+def static_compiler_rt : Flag<["-"], "static-compiler-rt">, Alias<static_libgcc>;
 def static_libstdcxx : Flag<["-"], "static-libstdc++">;
+def static_libcxx : Flag<["-"], "static-libc++">, Alias<static_libstdcxx>;
 def static : Flag<["-", "--"], "static">, Flags<[NoArgumentUnused]>;
 def std_default_EQ : Joined<["-"], "std-default=">;
 def std_EQ : Joined<["-", "--"], "std=">, Flags<[CC1Option]>,
   Group<CompileOnly_Group>, HelpText<"Language standard to compile for">,
   ValuesCode<[{
     const char *Values =
     #define LANGSTANDARD(id, name, lang, desc, features) name ","
     #define LANGSTANDARD_ALIAS(id, alias) alias ","