[Builtins] Implement __builtin_is_constant_evaluated for use in C++2a

Summary:
This patch implements `__builtin_is_constant_evaluated` as specifier by [P0595R2](https://wg21.link/p0595r2). It is built on the back of Bill Wendling's work for `__builtin_constant_p()`.

More tests to come, but early feedback is appreciated.

I plan to implement warnings for common mis-usages like those belowe in a following patch:
```
void foo(int x) {
  if constexpr (std::is_constant_evaluated())) { // condition is always `true`. Should use plain `if` instead.
   foo_constexpr(x);
  } else {
    foo_runtime(x);
  }
}
```



Reviewers: rsmith, MaskRay, bruno, void

Reviewed By: rsmith

Subscribers: dexonsmith, zoecarver, fdeazeve, kristina, cfe-commits

Differential Revision: https://reviews.llvm.org/D55500

llvm-svn: 359067

1 files changed, 2 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index aea99dbb..a5c0224 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -1783,7 +1783,8 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
   }
 
   llvm::Constant *constant = nullptr;
-  if (emission.IsConstantAggregate || D.isConstexpr()) {
+  if (emission.IsConstantAggregate || D.isConstexpr() ||
+      D.isUsableInConstantExpressions(getContext())) {
     assert(!capturedByInit && "constant init contains a capturing block?");
     constant = ConstantEmitter(*this).tryEmitAbstractForInitializer(D);
     if (constant && trivialAutoVarInit !=