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

[Fangrui Song via cfe-commits]

================
@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -Wunused -x c -verify %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -Wunused -x c++ -verify=expected,cxx %s
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+static int f(void) { return 42; }
+int g(void) __attribute__((alias("f")));
+
+static int foo [] = { 42, 0xDEAD }; // cxx-warning{{variable 'foo' is not needed and will not be emitted}}
+extern typeof(foo) bar __attribute__((unused, alias("foo")));
+
+static int (*resolver(void))(void) { return f; } // cxx-warning{{unused function 'resolver'}}
+int ifunc(void) __attribute__((ifunc("resolver")));
+
+static int __attribute__((overloadable)) f0(int x) { return x; }
+static float __attribute__((overloadable)) f0(float x) { return x; } // expected-warning{{unused function 'f0'}}
+int g0(void) __attribute__((alias("_ZL2f0i")));
+
+#ifdef __cplusplus
+static int f1() { return 42; }
+int g1(void) __attribute__((alias("_ZL2f1v")));
+}
+
+/// We demangle alias/ifunc target and mark all found functions as used.
+
+static int f2(int) { return 42; } // cxx-warning{{unused function 'f2'}}
+static int f2() { return 42; }
+int g2() __attribute__((alias("_ZL2f2v")));
+
+static int (*resolver1())() { return f; } // cxx-warning{{unused function 'resolver1'}}
+static int (*resolver1(int))() { return f; }
+int ifunc1() __attribute__((ifunc("_ZL9resolver1i")));
+
+/// We should report "unused function" for f3(int).
+namespace ns {
+static int f3(int) { return 42; } // cxx-warning{{unused function 'f3'}}
+static int f3() { return 42; } // cxx-warning{{unused function 'f3'}}
----------------
MaskRay wrote:

To support `ns::f3`, it seems that we need to use `LookupNestedNameSpecifierName` first to find `ns`, then find `f3`. The process is quite complex and may not fit into the specific `mark*` function.

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