Add TableGen ctags(1) emitter and helper script.

To use this in conjunction with exuberant ctags to generate a single
combined tags file, run tblgen first and then
  $ ctags --append [...]

Since some identifiers have corresponding definitions in C++ code,
it can be useful (if using vim) to also use cscope, and
  :set cscopetagorder=1
so that
  :tag X
will preferentially select the tablegen symbol, while
  :cscope find g X
will always find the C++ symbol.

Patch by Kevin Schoedel!

(a couple small formatting changes courtesy of clang-format)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177682 91177308-0d34-0410-b5e6-96231b3b80d8

utils/TableGen/TableGen.cpp

@@ -40,7 +40,8 @@ enum ActionType {
   GenTgtIntrinsic,
   PrintEnums,
   PrintSets,
-  GenOptParserDefs
+  GenOptParserDefs,
+  GenCTags
 };
 
 namespace {
@@ -82,6 +83,8 @@ namespace {
                                "Print expanded sets for testing DAG exprs"),
                     clEnumValN(GenOptParserDefs, "gen-opt-parser-defs",
                                "Generate option definitions"),
+                    clEnumValN(GenCTags, "gen-ctags",
+                               "Generate ctags-compatible index"),
                     clEnumValEnd));
 
   cl::opt<std::string>
@@ -161,6 +164,9 @@ bool LLVMTableGenMain(raw_ostream &OS, RecordKeeper &Records) {
     }
     break;
   }
+  case GenCTags:
+    EmitCTags(Records, OS);
+    break;
   }
 
   return false;