[Code Coverage] Add a tool to check test coverage of a patch

This script create a patch from the HEAD commit, extract modified or added source files, test case files
and source code lines, add coverage instrumentation for the affected source files, runs Lit tests,
and records which test cases cause each counter to be executed. Then report the number of test
cases executing the counter and the number of test cases executing the counter that are also changed
in some way by the patch. Thus providing developer the information of inadequately tested source lines.

Author: xgupta