[NFC] Introduce DiagRef and use it throughout the parser

The goal is to have a lightweight way to pass an unapplied
diagnostic to general routines.  Constructing a Diagnostic
is quite expensive as something we're potentially doing in
hot paths, as opposed to just when we're actually emitting
the diagnostic.  This design allows the expense to be delayed
until we need it.

I've also optimized the Diagnostic constructor to avoid
copying arguments unnecessarily; this is a relatively small
expense, since arguments are POD, but there's really no good
reason not to do it.

lib/SIL/Parser/ParseSIL.cpp

@@ -138,7 +138,7 @@ ParseSILModuleRequest::evaluate(Evaluator &evaluator,
 //===----------------------------------------------------------------------===//
 
 bool SILParser::parseSILIdentifier(Identifier &Result, SourceLoc &Loc,
-                                   const Diagnostic &D) {
+                                   DiagRef D) {
   switch (P.Tok.getKind()) {
   case tok::identifier:
   case tok::dollarident:
@@ -602,7 +602,7 @@ bool SILParser::parseSILQualifier(
   }
   result = parseName(Str);
   if (!result) {
-    P.diagnose(loc, Diagnostic(diag::unrecognized_sil_qualifier));
+    P.diagnose(loc, diag::unrecognized_sil_qualifier);
     return true;
   }
   return false;
@@ -2051,7 +2051,7 @@ parseAssignOrInitAssignments(llvm::SmallVectorImpl<unsigned> &assignments,
 // Returns true on error.
 static bool parseIndexList(Parser &P, StringRef label,
                            SmallVectorImpl<unsigned> &indices,
-                           const Diagnostic &parseIndexDiag) {
+                           DiagRef parseIndexDiag) {
   SourceLoc loc;
   // Parse `[<label> <integer_literal>...]`.
   if (P.parseToken(tok::l_square, diag::sil_autodiff_expected_lsquare,
@@ -7761,7 +7761,7 @@ parseRootProtocolConformance(Parser &P, SILParser &SP, Type ConformingTy,
 
   if (P.parseSpecificIdentifier(
           ModuleKeyword, KeywordLoc,
-          Diagnostic(diag::expected_tok_in_sil_instr, ModuleKeyword)) ||
+          {diag::expected_tok_in_sil_instr, {ModuleKeyword}}) ||
       SP.parseSILIdentifier(ModuleName, Loc, diag::expected_sil_value_name))
     return ProtocolConformanceRef();