PR18793: If we try to EnterTokenStream when our current lexer is a caching

PR18793: If we try to EnterTokenStream when our current lexer is a caching
lexer, add the token buffer underneath the caching lexer where possible and
push the tokens directly into the caching lexer otherwise. We previously
put the lexer into a corrupted state where we could not guarantee to provide
the tokens in the right order and would sometimes assert.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218333 91177308-0d34-0410-b5e6-96231b3b80d8