Correct the starting location for instantiations of field declarations which

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!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157311 91177308-0d34-0410-b5e6-96231b3b80d8