Change EmitConstantExpr to allow failure.

IRgen no longer relies on isConstantInitializer, instead we just try
to emit the constant. If that fails then in C we emit an error
unsupported (this occurs when Sema accepted something that it doesn't
know how to fold, and IRgen doesn't know how to emit) and in C++ we
emit a guarded initializer.

This ends up handling a few more cases, because IRgen was actually
able to emit some of the constants Sema accepts but can't Evaluate().
For example, PR3398.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64780 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed