Add support for LLVM Source Based Coverage Metrics

[bartlettroscoe]

Feature Request

How can we make CDash better?

LLVM Source-Based coverage tooling is way better than GCOV in many ways (and apparently has been so for many years to my surprise). See:

• https://clang.llvm.org/docs/SourceBasedCodeCoverage.html

CDash needs to add support for this coverage data as uploaded by CTest.

See matching CMake/CTest issue:

• https://gitlab.kitware.com/cmake/cmake/-/issues/26932

Things that might be new for CDash compared to other coverage formats:

• Produces coverage for each template instantiation separately and none for the parent template function (see llvm-project#134927: [llvm-cov] 'llvm-cov show' output for template functions with lots of instantiations is overwhelming)

NOTE: This would seem to be a duplicate of #859?