When formatting a C++-only declaration name, enable C++ mode in the formatter's

When formatting a C++-only declaration name, enable C++ mode in the formatter's
language options. This is not really ideal -- we should require the right
language options to be passed in, or not require language options to format a
name -- but it fixes a number of *obviously* wrong formattings. Patch by
Olivier Goffart!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199778 91177308-0d34-0410-b5e6-96231b3b80d8