Correct the starting location for instantiations of field declarations which

start with a cv-qualifier. DeclaratorDecl::getTypeSpecStartLoc() does not
produce the location of the first type-specifier (the cv-qualifier) in this
case, because we don't track source locations for cv-qualifiers.

No test here: I've not found a way to test this with a lit-style test, and
introducing a gtest test for this seems unwarranted. Suggestions welcome!

Patch by Daniel Jasper!

llvm-svn: 157311