Remove 24 instances of 'REQUIRES: shell'

Tests fall into one of the following categories:

- The requirement was unnecessary

- Additional quoting was required for backslashes in paths (see "sed -e
  's/\\/\\\\/g'") in the sanitizer tests.

- OpenMP used 'REQUIRES: shell' as a proxy for the test failing on
  Windows. Those tests fail there reliably, so use XFAIL instead.

I tried not to remove shell requirements that were added to suppress
flaky test failures, but if I screwed up, we can add it back as needed.

llvm-svn: 284793