Sprinkle LLVM_READONLY/READNONE on a few DeclAttribute accessors (#19237)

No intended functionality change. Probably no performance change
either, but might as well.

Author: jrose-apple

include/swift/AST/Attr.h

@@ -335,6 +335,7 @@ class DeclAttribute : public AttributeBase {
     UserInaccessible = 1ull << (unsigned(DeclKindIndex::Last_Decl) + 7),
   };
 
+  LLVM_READNONE
   static uint64_t getOptions(DeclAttrKind DK);
 
   uint64_t getOptions() const {
@@ -386,6 +387,7 @@ class DeclAttribute : public AttributeBase {
     return canAttributeAppearOnDecl(getKind(), D);
   }
 
+  LLVM_READONLY
   static bool canAttributeAppearOnDecl(DeclAttrKind DK, const Decl *D);
 
   /// Returns true if multiple instances of an attribute kind
@@ -439,6 +441,7 @@ class DeclAttribute : public AttributeBase {
     return isNotSerialized(getKind());
   }
 
+  LLVM_READNONE
   static bool canAttributeAppearOnDeclKind(DeclAttrKind DAK, DeclKind DK);
 
   /// Returns the source name of the attribute, without the @ or any arguments.