[Format] Do not use a global static value for EOF within ScopedMacroState.

ScopedMacroState injects its own EOF token under certain conditions, and the
returned token may be modified in several different locations. If multiple
reformat operations are started in different threads, then they will both see
the same fake EOF token, and may both try to modify it. This is a data race.

This bug was caught with tsan.

Reviewers: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47759

llvm-svn: 334801