Allow adding a value to a flag in diagnostics.

Summary:
This allows callers of Diags.Report() to append a value to the name of
the flag associated with the diagnostic. This is useful in cases like
the -Rpass flag, where we want the diagnostic to show the name of the
pass that matched the pattern. Instead of showing "... [-Rpass]", this
allows us to show "... [-Rpass=passname]".

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://reviews.llvm.org/D3441