add clang code coverage compile flags (#41103)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41103
add a CLANG_CODE_COVERAGE option to CMakeList. If the option is ON, add code coverage needed compile flags.
Test Plan:
Clone pytorch source code to local, modified these changes and builded it with `CLANG_CODE_COVERAGE ON` and `BUILD_TESTS ON`. Run a manual test and attach code coverage report.
{F243609020}
Reviewed By: malfet
Differential Revision: D22422513
fbshipit-source-id: 27a31395c31b5b5f4b72523954722771d8f61080
