[FileCheck] Document FILECHECK_OPTS in -help

[thopre]

llvm/utils/FileCheck/FileCheck.cpp

27–29

Do I read this right and the options in effect are the union of those specified in FILECHECK_OPTS and those on the command-line?

[jdenny]

llvm/utils/FileCheck/FileCheck.cpp

27–29

Yes.

Interestingly, FILECHECK_OPTS is parsed before command-line options. However, at the time I implemented FILECHECK_OPTS, the only option I found where the order of multiple occurrences mattered was -D, and it gives precedence to earlier occurrences. That's surely wrong, and I think multiple definitions for the same variable should be an error instead, but I haven't found time to propose something.

[thopre]

LGTM.

llvm/utils/FileCheck/FileCheck.cpp

27–29

Alright, LGTM then. I'm not sure about forbidding multiple definitions of the same variable, I think it makes sense to redefine a variable in a CHECK directive (eg. lots of tests with REG being defined, although --enable-var-scope should probably be used in that case). It makes less sense on the command-line for sure and I would agree making it an error.

[jdenny]

llvm/utils/FileCheck/FileCheck.cpp

27–29

I just meant on the command line. Thanks for the review!

llvm/utils/FileCheck/FileCheck.cpp

 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/WithColor.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/FileCheck.h"
 #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);
 
 static cl::opt<std::string>
     InputFilename("input-file", cl::desc("File to check (defaults to stdin)"),
                   cl::init("-"), cl::value_desc("filename"));
 
 static cl::list<std::string> CheckPrefixes(