Reland "[Coverage] Fix branch coverage merging in FunctionCoverageSummary::get()...

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

Originally landed in: 6400905a615282c83a2fc6e49e57ff716aa8b4de
Reverted in: 668dccc396da4f593ac87c92dc0eb7bc983b5762

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