[clang-tblgen] Add -print-records and -dump-json modes.

[simon_tatham]

Summary

Currently, if clang-tblgen is run without a mode option, it defaults
to the first mode in its 'enum Action', which happens to be
-gen-clang-attr-classes. I think it makes more sense for it to behave
the same way as llvm-tblgen, i.e. print a diagnostic dump if it's not
given any more specific instructions.

I've also added the same -dump-json that llvm-tblgen supports. This
means any tblgen command line (whether llvm- or clang-) can be
mechanically turned into one that processes the same input into JSON.

[nhaehnle]

LGTM

utils/TableGen/TableGen.cpp

 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Main.h"
 #include "llvm/TableGen/Record.h"
 
 using namespace llvm;
 using namespace clang;
 
 enum ActionType {
+  PrintRecords,
+  DumpJSON,
   GenClangAttrClasses,
   GenClangAttrParserStringSwitches,
   GenClangAttrSubjectMatchRulesParserStringSwitches,
   GenClangAttrImpl,
   GenClangAttrList,
   GenClangAttrSubjectMatchRuleList,
   GenClangAttrPCHRead,
   GenClangAttrPCHWrite,
enum ActionType {
   GenDataCollectors,
   GenTestPragmaAttributeSupportedAttributes
 };
 
 namespace {
 cl::opt<ActionType> Action(
     cl::desc("Action to perform:"),
     cl::values(
+        clEnumValN(PrintRecords, "print-records",
+                   "Print all records to stdout (default)"),
+        clEnumValN(DumpJSON, "dump-json",
+                   "Dump all records as machine-readable JSON"),
         clEnumValN(GenClangAttrClasses, "gen-clang-attr-classes",
                    "Generate clang attribute clases"),
         clEnumValN(GenClangAttrParserStringSwitches,
                    "gen-clang-attr-parser-string-switches",
                    "Generate all parser-related attribute string switches"),
         clEnumValN(GenClangAttrSubjectMatchRulesParserStringSwitches,
                    "gen-clang-attr-subject-match-rules-parser-string-switches",
                    "Generate all parser-related attribute subject match rule"

 
 cl::opt<std::string>
 ClangComponent("clang-component",
                cl::desc("Only use warnings from specified component"),
                cl::value_desc("component"), cl::Hidden);
 
 bool ClangTableGenMain(raw_ostream &OS, RecordKeeper &Records) {
   switch (Action) {
+  case PrintRecords:
+    OS << Records;           // No argument, dump all contents
+    break;
+  case DumpJSON:
+    EmitJSON(Records, OS);
+    break;
   case GenClangAttrClasses:
     EmitClangAttrClass(Records, OS);
     break;
   case GenClangAttrParserStringSwitches:
     EmitClangAttrParserStringSwitches(Records, OS);
     break;
   case GenClangAttrSubjectMatchRulesParserStringSwitches:
     EmitClangAttrSubjectMatchRulesParserStringSwitches(Records, OS);