Various fixes to clang-format's macro handling.

Some of this is still pretty rough (note the load of FIXMEs), but it is
strictly an improvement and fixes various bugs that were related to
macro processing but are also imporant in non-macro use cases.

Specific fixes:
- correctly puts espaced newlines at the end of the line
- fixes counting of white space before a token when escaped newlines are
  present
- fixes parsing of "trailing" tokens when eof() is hit
- puts macro parsing orthogonal to parsing other structure
- general support for parsing of macro definitions

Due to the fix to format trailing tokens, this change also includes a
bunch of fixes to the c-index tests.
[Driver] Add x86-64 target.