[ASTMatchers] fix isStaticStorageClass not matching definitions of forward declared functions

```c++
static void foo();
void foo() {}

struct A {
    static void bar();
};
void A::bar() {}
```

Both definitions refer to their previous declaration, but were not
considered `static`, because the matcher did not check the canonical
declaration.

Author: 5chmidti

clang/docs/LibASTMatchersReference.html

@@ -4683,8 +4683,10 @@ <h2 id="narrowing-matchers">Narrowing Matchers</h2>
   static int i = 0;
   extern int j;
   int k;
+  static void l();
+  void l() {}
 functionDecl(isStaticStorageClass())
-  matches the function declaration f.
+  matches the function declaration of f and l, and the definition of l.
 varDecl(isStaticStorageClass())
   matches the variable declaration i.
 </pre></td></tr>

clang/docs/ReleaseNotes.rst

@@ -1041,6 +1041,9 @@ AST Matchers
 
 - Ensure ``pointee`` matches Objective-C pointer types.
 
+- Fixed `isStaticStorageClass` not matching the definition if the definition was
+  not marked `static` as well.
+
 clang-format
 ------------
 

clang/include/clang/ASTMatchers/ASTMatchers.h

@@ -5418,15 +5418,17 @@ AST_POLYMORPHIC_MATCHER(isExternC, AST_POLYMORPHIC_SUPPORTED_TYPES(FunctionDecl,
 ///   static int i = 0;
 ///   extern int j;
 ///   int k;
+///   static void l();
+///   void l() {}
 /// \endcode
 /// functionDecl(isStaticStorageClass())
-///   matches the function declaration f.
+///   matches the function declaration of f and l, and the definition of l.
 /// varDecl(isStaticStorageClass())
 ///   matches the variable declaration i.
 AST_POLYMORPHIC_MATCHER(isStaticStorageClass,
                         AST_POLYMORPHIC_SUPPORTED_TYPES(FunctionDecl,
                                                         VarDecl)) {
-  return Node.getStorageClass() == SC_Static;
+  return Node.getCanonicalDecl()->getStorageClass() == SC_Static;
 }
 
 /// Matches deleted function declarations.

clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp

@@ -1826,6 +1826,14 @@ TEST_P(ASTMatchersTest, IsStaticStorageClass) {
   EXPECT_TRUE(notMatches("int i = 1;", varDecl(isStaticStorageClass())));
   EXPECT_TRUE(notMatches("extern int i;", varDecl(isStaticStorageClass())));
   EXPECT_TRUE(notMatches("void f() {}", functionDecl(isStaticStorageClass())));
+
+  if (!GetParam().isCXX())
+    return;
+
+  EXPECT_TRUE(matches("static void foo(); void foo() {}",
+                      functionDecl(isDefinition(), isStaticStorageClass())));
+  EXPECT_TRUE(matches("struct A { static void bar(); }; void A::bar() {}",
+                      cxxMethodDecl(isDefinition(), isStaticStorageClass())));
 }
 
 TEST_P(ASTMatchersTest, IsDefaulted) {