[clang-tidy]misc-const-correctness fix fake positive of function pointer (#162079)

Fix clang-tidy misc-const-correctness of function pointer, because
function pointer can't be declared as `const*const`

```
void function_pointer_basic() {
  void (*const fp)() = nullptr;
  fp();
}


test.cpp:2:3: warning: pointee of variable 'fp' of type 'void (*const)()' can be declared 'const' [misc-const-correctness]
    2 |   void (*const fp)() = nullptr;
      |   ^
      |                const 
```

---------

Co-authored-by: Congcong Cai <[email protected]>

Author: Changqing-JING

clang-tools-extra/clang-tidy/misc/ConstCorrectnessCheck.cpp

@@ -249,7 +249,8 @@ void ConstCorrectnessCheck::check(const MatchFinder::MatchResult &Result) {
       CheckValue();
     if (WarnPointersAsPointers) {
       if (const auto *PT = dyn_cast<PointerType>(VT)) {
-        if (!PT->getPointeeType().isConstQualified())
+        if (!PT->getPointeeType().isConstQualified() &&
+            !PT->getPointeeType()->isFunctionType())
           CheckPointee();
       }
       if (const auto *AT = dyn_cast<ArrayType>(VT)) {

clang-tools-extra/docs/ReleaseNotes.rst

@@ -328,7 +328,8 @@ Changes in existing checks
 
 - Improved :doc:`misc-const-correctness
   <clang-tidy/checks/misc/const-correctness>` check to avoid false
-  positives when pointers is tranferred to non-const references.
+  positives when pointers is tranferred to non-const references 
+  and avoid false positives of function pointer.
 
 - Improved :doc:`misc-header-include-cycle
   <clang-tidy/checks/misc/header-include-cycle>` check performance.

clang-tools-extra/test/clang-tidy/checkers/misc/const-correctness-pointer-as-pointers.cpp

@@ -48,6 +48,11 @@ void ignore_const_alias() {
   p_local0 = &a[1];
 }
 
+void function_pointer_basic() {
+  void (*const fp)() = nullptr;
+  fp();
+}
+
 void takeNonConstRef(int *&r);
 
 void ignoreNonConstRefOps() {