Add begin source location for the attributed statement created from PragmaLoopHint decorated loop

Summary:
Right now it is a '<invalid sloc>' for cases like this.
CounterCoverageMappingBuilder relies on the information to decide the
region for a attributed loop.

Fixes PR40971

Reviewers: ABataev, jdenny, lebedev.ri, aaron.ballman

Reviewed by: jdenny, aaron.ballman

Differential Revision: https://reviews.llvm.org/D80944