Throw the switch to exclusively use Evaluate (along with the small

helper isConstantInitializer) to check whether an initializer is 
constant.  This passes tests, but it's possible that it'll cause 
regressions with real-world code.

Future work:
1. The diagnostics obtained this way are lower quality at the moment; 
some work both here and in Evaluate is needed for accurate diagnostics.
2. We probably need some extra code when we're in -pedantic mode so we 
can strictly enforce the rules in C99 6.6p7.
3. Dead code cleanup (this should wait until after 2, because we might 
want to re-use some of the code).

llvm-svn: 65265