[FileCheck] Document FILECHECK_OPTS in -help

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D65707

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368787 91177308-0d34-0410-b5e6-96231b3b80d8

utils/FileCheck/FileCheck.cpp

@@ -24,6 +24,10 @@
 #include <cmath>
 using namespace llvm;
 
+static cl::extrahelp FileCheckOptsEnv(
+    "\nOptions are parsed from the environment variable FILECHECK_OPTS and\n"
+    "from the command line.\n");
+
 static cl::opt<std::string>
     CheckFilename(cl::Positional, cl::desc("<check-file>"), cl::Optional);