Un-revert "[Driver] Add -fdiagnostics-hotness-threshold"

Summary:
Un-revert https://reviews.llvm.org/D34868, but with a slight tweak to the
documentation to fix an error -- I had used the wrong syntax for a link.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306948 91177308-0d34-0410-b5e6-96231b3b80d8