[llvm-cov] Track function and instantiation coverage separately

These are distinct statistics which are useful to look at separately.

Example: say you have a template function "foo" with 5 instantiations
and only 3 of them are covered. Then this contributes (1/1) to the total
function coverage and (3/5) to the total instantiation coverage. I.e,
the old "Function Coverage" column has been renamed to "Instantiation
Coverage", and the new "Function Coverage" aggregates information from
the various instantiations of a function.

One benefit of making this switch is that the Line and Region coverage
columns will start making sense. Let's continue the example and assume
that the 5 instantiations of "foo" cover {2, 4, 6, 8, 10} out of 10
lines respectively. The new line coverage for "foo" is (10/10), not
(30/50).  The old scenario got confusing because we'd report that there
were more lines in a file than what was actually possible.

llvm-svn: 281875