Consider hidden decls for isUsed checks.

This fixes pr17624.

A FIXME from Richard Smith:

It seems to me that the root cause is that a per-Decl 'used' flag doesn't
really make much sense in the way we use it now. I think we should either track
whether that particular declaration is used (with isUsed scanning the entire
redecl chain), or we should only have one flag for the entire redeclaration
chain (perhaps by always looking at the flag on either the most recent decl or
the canonical decl). Modeling it as "is this declaration or any previous
declaration used" is weird, and requires contortions like the loop at the end
of Sema::MarkFunctionReferenced.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193202 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaDecl.cpp

@@ -2788,7 +2788,7 @@ bool Sema::MergeCompatibleFunctionDecls(FunctionDecl *New, FunctionDecl *Old,
     New->setPure();
 
   // Merge "used" flag.
-  New->setIsUsed(Old->isUsed(false));
+  New->setIsUsed(Old->getMostRecentDecl()->isUsed(false));
 
   // Merge attributes from the parameters.  These can mismatch with K&R
   // declarations.
@@ -3101,7 +3101,7 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
   }
 
   // Merge "used" flag.
-  New->setIsUsed(Old->isUsed(false));
+  New->setIsUsed(Old->getMostRecentDecl()->isUsed(false));
 
   // Keep a chain of previous declarations.
   New->setPreviousDecl(Old);

test/Sema/warn-variable-not-needed.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wall %s
+// expected-no-diagnostics
+
+static int a;
+int bar() {
+  extern int a;
+  return a;
+}
+static int a;

test/SemaCXX/warn-func-not-needed.cpp

@@ -42,3 +42,12 @@ namespace test4 {
     g<int>();
   }
 }
+
+namespace test4 {
+  static void func();
+  void bar() {
+    void func();
+    func();
+  }
+  static void func() {}
+}