Update the SARIF exporter to SARIF 2.1

This updates the SARIF exporter to produce SARIF 2.1 output. The bulk of the diffs come from two changes to SARIF:
* https://github.com/oasis-tcs/sarif-spec/issues/309
* https://github.com/oasis-tcs/sarif-spec/issues/179

Differential Revision: https://reviews.llvm.org/D65211

llvm-svn: 370068