SamplePGO - Add flag to check sampling coverage.

SamplePGO - Add flag to check sampling coverage.

This adds the flag -mllvm -sample-profile-check-coverage=N to the
SampleProfile pass. N is the percent of input sample records that the
user expects to apply.  If the pass does not use N% (or more) of the
sample records in the input, it emits a warning.

This is useful to detect some forms of stale profiles. If the code has
drifted enough from the original profile, there will be records that do
not match the IR anymore.

This will not detect cases where a sample profile record for line L is
referring to some other instructions that also used to be at line L.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251568 91177308-0d34-0410-b5e6-96231b3b80d8