[clang][Interp] consider "MS constexpr" functions as well

This implements the minimum amout of support for this feature
to get the test/AST/ms-constexpr.cpp test working. More has to
be added to get SemaCXX tests to work.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeEmitter.cpp

@@ -14,6 +14,7 @@
 #include "Opcode.h"
 #include "Program.h"
 #include "clang/AST/ASTLambda.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/Basic/Builtins.h"
 #include <type_traits>
@@ -116,7 +117,8 @@ Function *ByteCodeEmitter::compileFunc(const FunctionDecl *FuncDecl) {
   if (const auto *MD = dyn_cast<CXXMethodDecl>(FuncDecl))
     IsEligibleForCompilation = MD->isLambdaStaticInvoker();
   if (!IsEligibleForCompilation)
-    IsEligibleForCompilation = FuncDecl->isConstexpr();
+    IsEligibleForCompilation =
+        FuncDecl->isConstexpr() || FuncDecl->hasAttr<MSConstexprAttr>();
 
   // Compile the function body.
   if (!IsEligibleForCompilation || !visitFunc(FuncDecl)) {

clang/test/AST/ms-constexpr.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fms-compatibility -fms-compatibility-version=19.33 -std=c++20 -ast-dump -verify %s | FileCheck %s
+// RUN: %clang_cc1 -fms-compatibility -fms-compatibility-version=19.33 -std=c++20 -ast-dump -verify %s -fexperimental-new-constant-interpreter | FileCheck %s
 // expected-no-diagnostics
 
 // CHECK: used f1 'bool ()'