[llvm-cov[ Fix lcov coverage report contains functions from other compilation units.

Summary: Patch by Chuan Qiu (@eagleonhill).

Reviewers: Dor1s

Reviewed By: Dor1s

Subscribers: lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63571

llvm-svn: 364653