[Sema] Avoid instantiating templates only when UncompilableErrorOccurred

and FatalErrorOccurred are both set.

This fixes a crash that occurs when a warning promoted to a fatal error
leaves the AST in an incomplete state, and then later CFG analysis is
run on the incomplete AST.

rdar://problem/28558923

Differential Revision: https://reviews.llvm.org/D26166

llvm-svn: 285923