Minor optimization to constant evaluation: don't bother computing expr source

Minor optimization to constant evaluation: don't bother computing expr source
locations for diagnostics we're not going to emit, and don't track the subobject
designator outside C++11 (since we're not going to use it anyway).

This seems to give about a 0.5% speedup on 403.gcc/combine.c, but the results
were sufficiently noisy that I can't reject the null hypothesis.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152761 91177308-0d34-0410-b5e6-96231b3b80d8