[clang] Frontend components for the relative vtables ABI

This patch contains all of the clang changes from D72959.

- Generalize the relative vtables ABI such that it can be used by other targets.
- Add an enum VTableComponentLayout which controls whether components in the
  vtable should be pointers to other structs or relative offsets to those structs.
  Other ABIs can change this enum to restructure how components in the vtable
  are laid out/accessed.
- Add methods to ConstantInitBuilder for inserting relative offsets to a
  specified position in the aggregate being constructed.

See D72959 for background info.

Differential Revision: https://reviews.llvm.org/D77592

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 29e92ce..3aea916 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -3410,6 +3410,11 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.CompatibilityQualifiedIdBlockParamTypeChecking =
       Args.hasArg(OPT_fcompatibility_qualified_id_block_param_type_checking);
+
+  Opts.RelativeCXXABIVTables =
+      Args.hasFlag(OPT_fexperimental_relative_cxx_abi_vtables,
+                   OPT_fno_experimental_relative_cxx_abi_vtables,
+                   /*default=*/false);
 }
 
 static bool isStrictlyPreprocessorAction(frontend::ActionKind Action) {