Use the more general Evaluate infrastructure for folding constant DeclRefs

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++.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89934 91177308-0d34-0410-b5e6-96231b3b80d8