[clang][AST] ASTNameGenerator: A refactoring of CodegenNameGeneratorImpl (NFC).

This is a NFC refactor move of CodegenNameGeneratorImpl from clang::Index to
clang:AST (and rename to ASTNameGenerator). The purpose is to make the
highlevel mangling code more reusable inside of clang (say in places like clang
FrontendAction). This does not affect anything in CodegenNameGenerator, except
that CodegenNameGenerator will now use ASTNameGenerator (in AST).

Differential Revision: https://reviews.llvm.org/D63535

llvm-svn: 363878