Changes:
- initializer expressions of constexpr variable are now wraped in a ConstantExpr. this is mainly used for testing purposes. the old caching system has not yet been removed.
- Add all the missing Serialization and Importing for APValue.
- Improve dumping of APValue when ASTContext isn't available.
- Cleanup leftover from last patch.
- Add Tests for Import and serialization.
Differential Revision: https://reviews.llvm.org/D63640
Tyker