[sanitizer-coverage] allow -fsanitize-coverage=trace-pc w/o any other...

[sanitizer-coverage] allow -fsanitize-coverage=trace-pc w/o any other sanitizer and w/o ...=[func,bb,edge]. This makes this syntax a superset of the GCC's syntax

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261182 91177308-0d34-0410-b5e6-96231b3b80d8