[Sema] Warn about unused variables if we can constant evaluate the initializer.

[Sema] Warn about unused variables if we can constant evaluate the initializer.

If the variable construction can be constant evaluated it doesn't have
side effects, so removing it is always safe. We only try to evaluate
variables that are unused, there should be no impact on compile time.

Differential Revision: https://reviews.llvm.org/D38678

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315787 91177308-0d34-0410-b5e6-96231b3b80d8