AST: Mangle cast expression encoding more accurately

Don't mangle all casts in expressions as "cv", use the appropriate
encoding which corresponds to a specific cast.

This fixes PR21034.

Differential Revision: http://reviews.llvm.org/D5453

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218293 91177308-0d34-0410-b5e6-96231b3b80d8