[clang][AST] Refactoring ASTNameGenerator to use pimpl pattern (NFC).

The original pimpl pattern used between CodegenNameGenerator and
CodegenNameGeneratorImpl did a good job of hiding DataLayout making it so that
users of CodegenNameGenerator did not need to link with llvm core.  This is an
NFC change to neatly wrap ASTNameGenerator in a pimpl.

Differential Revision: https://reviews.llvm.org/D63584

llvm-svn: 363908