[clang][Driver] Expose -fexperimental-relative-c++-abi-vtables flag

Initially, this flag was meant to only be used through cc1 and not directly
through the clang driver. However, we accidentally ended up using this flag
as a driver flag already for selecting multilibs within the fuchsia toolchain.
We're currently in an awkward state where it's only accepted as a driver flag
when targeting Fuchsia, and all other instances it can only be added via
-Xclang. Since we're ready to use this in Fuchsia, we can just expose this to
the driver for simplicity.

Differential Revision: https://reviews.llvm.org/D98375

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4869,6 +4869,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.AddLastArg(CmdArgs, options::OPT_ffine_grained_bitfield_accesses,
                   options::OPT_fno_fine_grained_bitfield_accesses);
 
+  Args.AddLastArg(CmdArgs, options::OPT_fexperimental_relative_cxx_abi_vtables,
+                  options::OPT_fno_experimental_relative_cxx_abi_vtables);
+
   // Handle segmented stacks.
   if (Args.hasArg(options::OPT_fsplit_stack))
     CmdArgs.push_back("-split-stacks");

clang/test/Driver/relative-vtables-flag.cpp

@@ -0,0 +1,7 @@
+// RUN: %clangxx --target=aarch64-unknown-fuchsia -fexperimental-relative-c++-abi-vtables -c %s -### 2>&1 | FileCheck %s --check-prefix=RV
+// RUN: %clangxx --target=aarch64-unknown-fuchsia -fno-experimental-relative-c++-abi-vtables -c %s -### 2>&1 | FileCheck %s --check-prefix=NO-RV
+// RUN: %clangxx --target=aarch64-unknown-fuchsia -c %s -### 2>&1 | FileCheck %s --check-prefix=NO-RV
+// RUN: %clangxx --target=aarch64-unknown-linux-gnu -c %s -### 2>&1 | FileCheck %s --check-prefix=NO-RV
+
+// RV: "-fexperimental-relative-c++-abi-vtables"
+// NO-RV-NOT: "-fexperimental-relative-c++-abi-vtables"