[Sema] Move Diags.isIgnored() checks off hot paths, it's not free. NFC

[sammccall]

Summary

This speeds up clangd's buildAST() (i.e. parsing with a preamble) by 5% on
clangd/AST.cpp, by avoiding filling up the diagnostic state map with entries for
all the files where templates are being instantiated from.

(I would assume it has a similar effect on PCH and modules compiles).

This approach is obviously pretty fragile, and we should find ways to make
isIgnored() cheaper instead. But these changes in particular don't seem to make
the code worse in any case.

clang/lib/Sema/Sema.cpp

void Sema::diagnoseNullableToNonnullConversion(QualType DstType,
 
   Optional<NullabilityKind> TypeNullability = DstType->getNullability(Context);
   if (!TypeNullability || *TypeNullability != NullabilityKind::NonNull)
     return;
 
   Diag(Loc, diag::warn_nullability_lost) << SrcType << DstType;
 }
 
-void Sema::diagnoseZeroToNullptrConversion(CastKind Kind, const Expr* E) {
-  if (Diags.isIgnored(diag::warn_zero_as_null_pointer_constant,
-                      E->getBeginLoc()))
-    return;
+void Sema::diagnoseZeroToNullptrConversion(CastKind Kind, const Expr *E) {
   // nullptr only exists from C++11 on, so don't warn on its absence earlier.
   if (!getLangOpts().CPlusPlus11)
     return;
 
   if (Kind != CK_NullToPointer && Kind != CK_NullToMemberPointer)
     return;
   if (E->IgnoreParenImpCasts()->getType()->isNullPtrType())
     return;
 
+  if (Diags.isIgnored(diag::warn_zero_as_null_pointer_constant,
+                      E->getBeginLoc()))
+    return;
+
   // Don't diagnose the conversion from a 0 literal to a null pointer argument
   // in a synthesized call to operator<=>.
   if (!CodeSynthesisContexts.empty() &&
       CodeSynthesisContexts.back().Kind ==
           CodeSynthesisContext::RewritingOperatorAsSpaceship)
     return;
 
   // If it is a macro from system header, and if the macro name is not "NULL",