[Coverage] Adjust skipped regions only if {Prev,Next}TokLoc is in the same...

[Coverage] Adjust skipped regions only if {Prev,Next}TokLoc is in the same file as regions' {start, end}Loc

Fix a bug if {Prev, Next}TokLoc is in different file from skipped regions' {start, end}Loc

Differential Revision: https://reviews.llvm.org/D86116