[lit] Add support to print paths relative to CWD in the test summary report (#154317)

This patch adds a -r|--relative-paths option to llvm-lit, which when
enabled will print test case names using paths relative to the current
working directory. The legacy default without that option is that test
cases are identified using a path relative to the test suite.

Only the summary report is impacted. That normally include failed tests,
unless unless options such as --show-pass.

Background to this patch was the discussion here

https://discourse.llvm.org/t/test-case-paths-in-llvm-lit-output-are-lacking-the-location-of-the-test-dir-itself/87973
with a goal to making it easier to copy-n-paste the path to the failing
test cases.

Examples showing difference in "Passed Tests" and "Failed Tests":

 > llvm-lit --show-pass test/Transforms/Foo
 PASS: LLVM :: Transforms/Foo/test1.txt (1 of 2)
 FAIL: LLVM :: Transforms/Foo/test2.txt (2 of 2)
 Passed Tests (1):
   LLVM :: Transforms/Foo/test1.txt
 Failed Tests (1):
   LLVM :: Transforms/Foo/test2.txt

 > llvm-lit --show-pass --relative-paths test/Transforms/Foo
 PASS: LLVM :: Transforms/Foo/test1.txt (1 of 2)
 FAIL: LLVM :: Transforms/Foo/test2.txt (2 of 2)
 Passed Tests (1):
   test/Transforms/Foo/test1.txt
 Failed Tests (1):
   test/Transforms/Foo/test2.txt