[FileCheck] Don't propagate FILECHECK_DUMP_INPUT_ON_FAILURE and
FILECHECK_OPTS into environment for FileCheck tests.
This fixes the following FileCheck tests:
- FileCheck/dump-input-enable.txt
- FileCheck/match-full-lines.txt
when FILECHECK_DUMP_INPUT_ON_FAILURE is set in the environment.
By default llvm-lit propagates FILECHECK_DUMP_INPUT_ON_FAILURE and
FILECHECK_OPTS from llvm-lit's environment into the test environment.
Unfortunately this can break FileCheck's tests because they expect that
these environment variables not to be set.
rdar://problem/47176262