llvm-cov: Allow excluding functions from show and report

[rkjnsn]

I have some test functions I would like to exclude from the stats summary, html, and exported lcov data. However, I only see options to include specific functions / function patterns, not exclude. Additionally, report and export don't seem to honor --name-regex= despite it appearing in their --help output, and using --name-regex= with show changes the output to display individual functions rather than source files, whereas I would like to exclude specific functions while still keeping the output in source-file mode.

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 be omitted from coverage analysis entirely.)