[llvm] Add include guards to LLVMOption TableGen

[modocache]

Summary

Add include guards that allow multiple includes of OptParser.td. This
is helpful when defining multiple sets of options in multiple files.
For example, a user could define a HelpOptions.td file that defines
only -h and --help:

// HelpOptions.td
include "llvm/Option/OptParser.td"

def HelpOptionGroup : OptionGroup<"Help Options">;
def help : Flag<["--", "-"], "help">, Group<HelpOptionGroup>,
           Flags<[]>;
def : Flag<["-"], "h">, Group<HelpOptionGroup>, Flags<[]>, Alias<help>;

This file could then be included into any TableGen file that wishes to
define these options:

// MyOptions.td
include "llvm/Option/OptParser.td"

def MyOptionGroup : OptionGroup<"My Options">;
// ...define my options.

// And also define `-h` and `--help`:
include "HelpOptions.td"

This currently isn't possible, because this would result in
OptParser.td being included twice. Alternatively, the include of
OptParser.td in the HelpOptions.td example above could be removed,
but then llvm-tblgen --gen-opt-parser-defs HelpOptions.td would fail
(because the OptionGroup and Option records are defined in
OptParser.td).

[rriddle]

Seems like an objective improvement for enabling self-contained tablegen files, and shouldn't negatively impact any existing flow AFAICT.

llvm/include/llvm/Option/OptParser.td

 //===--- OptParser.td - Common Option Parsing Interfaces ------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the common interfaces used by the option parsing TableGen
 //  backend.
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_OPTION_OPTPARSER_TD
+#define LLVM_OPTION_OPTPARSER_TD
+
 // Define the kinds of options.
 
 class OptionKind<string name, int precedence = 0, bit sentinel = false> {
   string Name = name;
   // The kind precedence, kinds with lower precedence are matched first.
   int Precedence = precedence;
   // Indicate a sentinel option.
   bit Sentinel = sentinel;

 class ValueExtractor<code extractor> { code ValueExtractor = extractor; }
 
 // Predefined options.
 
 // FIXME: Have generator validate that these appear in correct position (and
 // aren't duplicated).
 def INPUT : Option<[], "<input>", KIND_INPUT>;
 def UNKNOWN : Option<[], "<unknown>", KIND_UNKNOWN>;
+
+#endif // LLVM_OPTION_OPTPARSER_TD