llvm-cov: Allow excluding functions from show and report

[rkjnsn]

I have some test functions I would like to exclude from the stats summary. However, show only has an option to include functions by regex, and report lacks any method to filter by function name at all.

llvm-profdata has both --function= and --no-function=, and it would be nice if llvm-cov had similar. (Passing --no-function= to llvm-profdata doesn't help in this case, as that causes the functions to be shown as uncovered by llvm-cov rather than to be omitted entirely.)