[clang] Improve Serialization/Imporing/Dumping of APValues

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