Use the more general Evaluate infrastructure for folding constant DeclRefs

instead of checking explicitly for an EnumConstantDecl.  This folds references
to constant integer VarDecls, which has two benefits:

1. Slightly smaller emitted code from emitting a constant instead of a load,
and skipping emitting some constant globals.
2. Some code forgets to define static constant member variables; emitting a
load instead of the value in that case leads to a link error. (Such programs
are technically not well-formed, but in practice build with g++.)

llvm-svn: 89934