[llvm-cov] Allow branch coverage to be skipped when exporting for LCOV

This small patch adds a '--skip-branches' option to the llvm-cov export
options. This option allows branch coverage information to be skipped from the
exported LCOV directives if it's not needed. For now, this only works when
exporting LCOV (which is noted in the option description), but it can be
extended for JSON later if it makes sense.

Differential Revision: https://reviews.llvm.org/D135986