Fix sarif-multi-diagnostic-test.c failure on Windows

[mgrang]

Some of the llvm/clang unit tests are non-portable to Windows because they were
mainly written for Linux by the community and somehow made to work on Windows.
The issue with the sarif-multi-diagnostics test seems to be due to grep. Due to
the presence of a Unicode string in this test grep considers this file as a
binary file instead of a text file and hence errors out with the following
message:
Binary file (standard input) matches

So we need to tell grep to treat this file as a text file for comparison
purposes. This can be done via the -a flag to grep. With this fix the test now
passes on ADO. The git log commit messages for the file
clang/test/Analysis/lit.local.cfg which I fixed mentions the non-portable
nature of these tests.

[kkjeer]

LGTM. Thanks for fixing this!

[sulekhark]

LGTM. Thank you!