[Clang][NFC] Some `const` for `IdentifierInfo *`s feeding `DeclarationName`

`DeclarationName` already takes `const IdentifierInfo *`. Propagate the
`const` outward to various APIs.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D122261
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 926426b..1246287 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -1041,7 +1041,7 @@
   };
 
   VarDecl(Kind DK, ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
-          SourceLocation IdLoc, IdentifierInfo *Id, QualType T,
+          SourceLocation IdLoc, const IdentifierInfo *Id, QualType T,
           TypeSourceInfo *TInfo, StorageClass SC);
 
   using redeclarable_base = Redeclarable<VarDecl>;
@@ -1071,8 +1071,8 @@
 
   static VarDecl *Create(ASTContext &C, DeclContext *DC,
                          SourceLocation StartLoc, SourceLocation IdLoc,
-                         IdentifierInfo *Id, QualType T, TypeSourceInfo *TInfo,
-                         StorageClass S);
+                         const IdentifierInfo *Id, QualType T,
+                         TypeSourceInfo *TInfo, StorageClass S);
 
   static VarDecl *CreateDeserialized(ASTContext &C, unsigned ID);