[AST] Fix crash in evaluation of OpaqueExpr in Clangd

The code mistakenly returns the value as evaluated whenever the
temporary is allocated, but not yet evaluated.

I could only reproduce in Clangd and could not come up with an example
that would crash the compiler. Clangd runs more evaluations for hover
than the language would allow.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D158985