[-Wunsafe-buffer-usage] Do not warn about class methods with libc function names

This commit fixes the false positive that C++ class methods with libc
function names would be false warned about. For example,

```
struct T {void strcpy() const;};
void test(const T& t) {  str.strcpy(); // no warn }
```

rdar://156264388

Author: ziqingluo-90

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -1925,7 +1925,7 @@ class UnsafeLibcFunctionCallGadget : public WarningGadget {
     if (!CE || !CE->getDirectCallee())
       return false;
     const auto *FD = dyn_cast<FunctionDecl>(CE->getDirectCallee());
-    if (!FD)
+    if (!FD || isa<CXXMethodDecl>(FD))
       return false;
     auto isSingleStringLiteralArg = false;
     if (CE->getNumArgs() == 1) {

clang/test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp

@@ -155,3 +155,21 @@ void ff(char * p, char * q, std::span<char> s, std::span<char> s2) {
   wcscpy_s();
 #pragma clang diagnostic pop
 }
+
+
+namespace CXXMethodNoWarn {
+  struct StrBuff
+  {
+    void strcpy() const;
+    void strcpy(char* dst) const;
+    void Strcpy(char* dst) const;
+  };
+
+  void test(const StrBuff& str)
+  {
+    char buff[64];
+    str.strcpy();
+    str.strcpy(buff);
+    str.Strcpy(buff);
+  }
+} // namespace CXXMethodNoWarn