[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@363908
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