Refactoring of constant expression evaluator

This introduces a generic base class for the expression evaluator
classes, which handles a few common expression types which were
previously handled separately in each class.  Also, the expression
evaluator now uses ConstStmtVisitor.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131281 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed