[syntax] Introduce a TokenManager interface.

TokenManager defines Token interfaces for the clang syntax-tree. This is the level
of abstraction that the syntax-tree should use to operate on Tokens.

It decouples the syntax-tree from a particular token implementation (TokenBuffer
previously).  This enables us to use a different underlying token implementation
for the syntax Leaf node -- in clang pseudoparser, we want to produce a
syntax-tree with its own pseudo::Token rather than syntax::Token.

Differential Revision: https://reviews.llvm.org/D128411