[clang] [Sema] Mark alias/ifunc targets used and consider mangled names (PR #87130)

[Fangrui Song via cfe-commits]

================
@@ -1,7 +1,35 @@
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -Wunneeded-internal-declaration -x c -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -Wunused -x c -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -Wunused -x c++ -verify %s
+
+#ifdef __cplusplus
+extern "C" {
+#else
 // expected-no-diagnostics
+#endif
 static int f(void) { return 42; }
 int g(void) __attribute__((alias("f")));
 
 static int foo [] = { 42, 0xDEAD };
 extern typeof(foo) bar __attribute__((unused, alias("foo")));
+
+static int (*resolver(void))(void) { return f; }
+int ifunc(void) __attribute__((ifunc("resolver")));
+
+#ifdef __cplusplus
+}
+
+/// We demangle alias/ifunc target and mark all found functions as used.
+static int f1(int) { return 42; }
+static int f1(void) { return 42; }
+int g1(void) __attribute__((alias("_ZL2f1v")));
+
+static int (*resolver1(void))(void) { return f; }
+static int (*resolver1(int))(void) { return f; }
+int ifunc1(void) __attribute__((ifunc("_ZL9resolver1v")));
+
+namespace ns {
+static int f2(int) { return 42; } // expected-warning{{unused function 'f2'}}
+static int f2(void) { return 42; } // expected-warning{{unused function 'f2'}}
----------------
MaskRay wrote:

`c++filt -p <<< _ZNL2nsL2f2Ev => ns::f2`. We attempt to find `ns::f2` in the scope and will find nothing (we should use `f2`).

This weakness is mentioned in the description as "... does not handle namespace scope names"

https://github.com/llvm/llvm-project/pull/87130