[Coverage][llvm-cov] Correctly export branch coverage in LCOV format

Commit 9f2967bc introduced support for
branch coverage including export to the LCOV format.

This commit corrects the LCOV field name for branches from BFH to BRH.
The mistake seems to have slipped in as typo because the correct field
name BRH is used in the comment section at the beginning of the file.

Differential Revision: https://reviews.llvm.org/D108358