isEvaluatable() implies a constant context.

isEvaluatable() implies a constant context.

Assume that we're in a constant context if we're asking if the expression can
be compiled into a constant initializer. This fixes the issue where a
__builtin_constant_p() in a compound literal was diagnosed as not being
constant, even though it's always possible to convert the builtin into a
constant.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@347512 91177308-0d34-0410-b5e6-96231b3b80d8