[LIT] Added an option to llvm-lit to emit the necessary test coverage data, divided per test case

This patch is the first part of https://llvm.org/OpenProjects.html#llvm_patch_coverage.

We have first define a new variable LLVM_TEST_COVERAGE which when set, pass --per-test-coverage option to
llvm-lit which will help in setting a unique value to LLVM_PROFILE_FILE for each RUN. So for example
coverage data for test case llvm/test/Analysis/AliasSet/memtransfer.ll will be emitted as
build/test/Analysis/AliasSet/memtransfer0.profraw

Reviewed By: hnrklssn

Differential Revision: https://reviews.llvm.org/D154280