Propagate SourceLocations through to get a Loc on float_cast_overflow

Summary:
float_cast_overflow is the only UBSan check without a source location attached.
This patch propagates SourceLocations where necessary to get them to the
EmitCheck() call.

Reviewers: rsmith, ABataev, rjmccall

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11757

llvm-svn: 244568