[FileCheck] Don't propagate `FILECHECK_DUMP_INPUT_ON_FAILURE` and
`FILECHECK_OPTS` into environment for FileCheck tests.

Summary:

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

Reviewers: jdenny, probinson, george.karpenkov

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56541

llvm-svn: 350850