[SourceMgr][FileCheck] Obey -color by extending WithColor

While this change specifically targets FileCheck, it affects any tool
using the same SourceMgr facilities.

Previously, -color was documented in FileCheck's -help output, but
-color had no effect.  Now, -color obeys its documentation: it forces
colors to be used in FileCheck diagnostics even when stderr is not a
terminal.

-color is especially helpful when combined with FileCheck's -v, which
can produce a long series of diagnostics that you might wish to pipe
to a pager, such as less -R.  The WithColor extensions here will also
help to clean up color usage in FileCheck's annotated dump of input,
which is proposed in D52999.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D53419

llvm-svn: 344930