[Coverage] Fix branch coverage merging in FunctionCoverageSummary::get() for instantiation

Fix branch coverage merging in FunctionCoverageSummary::get() for instantiation
groups.

This change corrects the implementation for the branch coverage summary to do
the same thing for branches that is done for lines and regions.  That is,
across function instantiations in an instantiation group, the maximum branch
coverage found in any of those instantiations is returned, with the total
number of branches being the same across instantiations.

Differential Revision: https://reviews.llvm.org/D102193

(cherry picked from commit eccb9251)