[clang-format] Add a MacroExpander.

Summary:
The MacroExpander allows to expand simple (non-resursive) macro
definitions from a macro identifier token and macro arguments. It
annotates the tokens with a newly introduced MacroContext that keeps
track of the role a token played in expanding the macro in order to
be able to reconstruct the macro expansion from an expanded (formatted)
token stream.

Made Token explicitly copy-able to enable copying tokens from the parsed
macro definition.

Reviewers: sammccall

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83296