[FileCheck] Make FILECHECK_OPTS useful for its test suite

Without this patch, `FILECHECK_OPTS` isn't propagated to FileCheck's
test suite so that `FILECHECK_OPTS` doesn't inadvertently affect test
results by affecting the output of FileCheck calls under test.  As a
result, `FILECHECK_OPTS` is useless for debugging FileCheck's test
suite.

In `llvm/test/FileCheck/lit.local.cfg`, this patch provides a new
subsitution, `%ProtectFileCheckOutput`, to address this problem for
both `FILECHECK_OPTS` and the deprecated
`FILECHECK_DUMP_INPUT_ON_FAILURE`.  The rest of the patch uses
`%ProtectFileCheckOutput` throughout the test suite

Fixes PR40284.

Reviewed By: probinson, thopre

Differential Revision: https://reviews.llvm.org/D65121