[WebAssembly] Don't set UseARMGuardVarABI, just use the default (64-bit)

[sbc100]

Summary

This fixes a disagreement between libc++abi and clang about the width
of the guard variable to use. Currnently libc++abi clobbers memory
because it thinks the region is 64-bit on all non-arm platforms.

If there is some reason to prefer 32-bit we can instead update
libc++abi.

Fixes PR42680

[sbc100]

We are going to change libc++abi instead: https://reviews.llvm.org/D64961

clang/lib/CodeGen/ItaniumCXXABI.cpp

public:
 
   // ARM64 libraries are prepared for non-unique RTTI.
   bool shouldRTTIBeUnique() const override { return false; }
 };
 
 class WebAssemblyCXXABI final : public ItaniumCXXABI {
 public:
   explicit WebAssemblyCXXABI(CodeGen::CodeGenModule &CGM)
-      : ItaniumCXXABI(CGM, /*UseARMMethodPtrABI=*/true,
-                      /*UseARMGuardVarABI=*/true) {}
+      : ItaniumCXXABI(CGM, /*UseARMMethodPtrABI=*/true {}
   void emitBeginCatch(CodeGenFunction &CGF, const CXXCatchStmt *C) override;
 
 private:
   bool HasThisReturn(GlobalDecl GD) const override {
     return isa<CXXConstructorDecl>(GD.getDecl()) ||
            (isa<CXXDestructorDecl>(GD.getDecl()) &&
             GD.getDtorType() != Dtor_Deleting);
   }