[8.0 Regression] Fix handling of `__builtin_constant_p` inside template arguments...

[8.0 Regression] Fix handling of `__builtin_constant_p` inside template arguments, enumerators, case statements, and the enable_if attribute.

Summary:
The following code is accepted by Clang 7 and prior but rejected by the upcoming 8 release and in trunk [1]

```
// error {{never produces a constant expression}}
void foo(const char* s) __attribute__((enable_if(__builtin_constant_p(*s) == false, "trap"))) {}
void test() { foo("abc"); }
```

Prior to Clang 8, the call to `__builtin_constant_p` was a constant expression returning false. Currently, it's not a valid constant expression.

The bug is caused because we failed to set `InConstantContext` when attempting to evaluate unevaluated constant expressions.

[1]  https://godbolt.org/z/ksAjmq

Reviewers: rsmith, hans, sbenza

Reviewed By: rsmith

Subscribers: kristina, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59038

llvm-svn: 355743