[libsanitizer] Add descriptions for the common flags.
Use new(allocator_for_flags) instead of allocator_for_flags.Allocate()
Fix the description output format a bit.
llvm-profdata: Implement show command

The `llvm-profdata show` command summarizes a profdata file's contents
in a human readable format.
Refactor llvm/test/lit.cfg to use lit.util.which.
Placate -Wunreachable-code by removing unnecessary logic to handle NUM_OPENMP_DEFAULT_KINDS <= 1.
Convert test to using cfi.

An unnamed global in llvm still produces a regular symbol.
ProfileData: Introduce InstrProfWriter using the naive text format

This isn't a format we'll want to write out in practice, but moving it
to the writer library simplifies llvm-profdata and isolates it from
further changes to the format.

This also allows us to update the tests to not rely on the text output
format.