Extend LLVM IR WME to use thunks for cross-module witness method calls. (#39528)

This enables optimizing / dead-stripping of witness methods across modules at
LTO time.

- Under -internalize-at-link, restrict visibility of wtables to linkage unit.
- Emit thunks for cross-module wcalls when WME is enabled.
- Use thunks for wcalls across modules when WME is enabled.
- Adjust TBDGen to account for witness method thunks when WME is enabled.
- Add an IR test to check that thunks are used when doing cross-module calls.
- Add an end-to-end test case for cross-module WME.

lib/Frontend/CompilerInvocation.cpp

@@ -1542,6 +1542,7 @@ static bool ParseTBDGenArgs(TBDGenOptions &Opts, ArgList &Args,
   Opts.IsInstallAPI = Args.hasArg(OPT_tbd_is_installapi);
 
   Opts.VirtualFunctionElimination = Args.hasArg(OPT_enable_llvm_vfe);
+  Opts.WitnessMethodElimination = Args.hasArg(OPT_enable_llvm_wme);
 
   if (const Arg *A = Args.getLastArg(OPT_tbd_compatibility_version)) {
     Opts.CompatibilityVersion = A->getValue();