[lit] Protect full test suite from FILECHECK_OPTS

lit's test suite calls lit multiple times for various sample test
suites.  `FILECHECK_OPTS` is safe for FileCheck calls in lit's test
suite.  It's not safe for FileCheck calls in the sample test suites,
whose output affects the results of lit's test suite.

Without this patch, only one such sample test suite is protected from
`FILECHECK_OPTS`, and currently `shtest-shell.py` breaks with
`FILECHECK_OPTS=-vv`.  Moreover, it's hard to predict the future,
especially false passes.  Thus, this patch protects all existing and
future sample test suites from `FILECHECK_OPTS` (and the deprecated
`FILECHECK_DUMP_INPUT_ON_FAILURE`).

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D65156

llvm/utils/lit/tests/lit.cfg

@@ -52,10 +52,17 @@ for attribute in ('llvm_tools_dir', 'lit_tools_dir'):
     if directory:
         llvm_config.with_environment('PATH', directory, append_path=True)
 
+# This test suite calls %{lit} to test lit's behavior for the sample test
+# suites in %{inputs}.  This test suite's results are then determined in part
+# by %{lit}'s textual output, which includes the output of FileCheck calls
+# within %{inputs}'s test suites.  Thus, %{lit} clears environment variables
+# that can affect FileCheck's output.
 config.substitutions.append(('%{inputs}', os.path.join(
     config.test_source_root, 'Inputs')))
-config.substitutions.append(('%{lit}', "%%{python} %s" % (
-    os.path.join(lit_path, 'lit.py'),)))
+config.substitutions.append(('%{lit}',
+    "{env} %{{python}} {lit}".format(
+        env="env -u FILECHECK_OPTS -u FILECHECK_DUMP_INPUT_ON_FAILURE",
+        lit=os.path.join(lit_path, 'lit.py'))))
 config.substitutions.append(('%{python}', '"%s"' % (sys.executable)))
 
 # Enable coverage.py reporting, assuming the coverage module has been installed

llvm/utils/lit/tests/shtest-run-at-line.py

@@ -1,7 +1,7 @@
 # Check that -vv makes the line number of the failing RUN command clear.
 # (-v is actually sufficient in the case of the internal shell.)
 #
-# RUN: env -u FILECHECK_OPTS not %{lit} -j 1 -vv %{inputs}/shtest-run-at-line > %t.out
+# RUN: not %{lit} -j 1 -vv %{inputs}/shtest-run-at-line > %t.out
 # RUN: FileCheck --input-file %t.out %s
 #
 # END.