[FileCheck] Parse command-line options from FILECHECK_OPTS
This feature makes it easy to tune FileCheck diagnostic output when
running the test suite via ninja, a bot, or an IDE. For example:
```
$ FILECHECK_OPTS='-color -v -dump-input-on-failure' \
LIT_FILTER='OpenMP/for_codegen.cpp' ninja check-clang \
| less -R
```
Reviewed By: probinson
Differential Revision: https://reviews.llvm.org/D53517
llvm-svn: 346272
diff --git a/llvm/docs/CommandGuide/FileCheck.rst b/llvm/docs/CommandGuide/FileCheck.rst
index 830b1e0..6581b33 100644
--- a/llvm/docs/CommandGuide/FileCheck.rst
+++ b/llvm/docs/CommandGuide/FileCheck.rst
@@ -24,6 +24,9 @@
OPTIONS
-------
+Options are parsed from the environment variable ``FILECHECK_OPTS``
+and from the command line.
+
.. option:: -help
Print a summary of command line options.