Introduce a caching mechanism for macro expanded tokens.

Previously macro expanded tokens were added to Preprocessor's bump allocator and never released,
even after the TokenLexer that were lexing them was finished, thus they were wasting memory.
A very "useful" boost library was causing clang to eat 1 GB just for the expanded macro tokens.

Introduce a special cache that works like a stack; a TokenLexer can add the macro expanded tokens
in the cache, and when it finishes, the tokens are removed from the end of the cache.

Now consumed memory by expanded tokens for that library is ~ 1.5 MB.

Part of rdar://9327049.

llvm-svn: 134105