[OpenMP] Add scaffolding for negative runtime tests

Without this patch, the openmp project's test suites do not appear to
have support for negative tests.  However, D78170 needs to add a test
that an expected runtime failure occurs.

This patch makes `not` visible in all of the openmp project's test
suites.  In all but `libomptarget/test`, it should be possible for a
test author to insert `not` before a use of the lit substitution for
running a test program.  In `libomptarget/test`, that substitution is
target-specific, and its value is `echo` when the target is not
available.  In that case, inserting `not` before a lit substitution
would expect an `echo` fail, so this patch instead defines a separate
lit substitution for expected runtime fails.

Reviewed By: jdoerfert, Hahnfeld

Differential Revision: https://reviews.llvm.org/D78566

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