[fuzzer] fix clang-cl build fuzzer lit test failure (#112339)

The `check-fuzzer` runs fine with cl build llvm, but the following lit
tests fail with clang-cl build llvm
```
********************
Timed Out Tests (2):
  libFuzzer-x86_64-default-Windows :: fork-ubsan.test
  libFuzzer-x86_64-default-Windows :: fuzzer-oom.test

********************
Failed Tests (22):
  libFuzzer-x86_64-default-Windows :: acquire-crash-state.test
  libFuzzer-x86_64-default-Windows :: cross_over_copy.test
  libFuzzer-x86_64-default-Windows :: cross_over_insert.test
  libFuzzer-x86_64-default-Windows :: exit_on_src_pos.test
  libFuzzer-x86_64-default-Windows :: fuzzer-alignment-assumption.test
  libFuzzer-x86_64-default-Windows :: fuzzer-implicit-integer-sign-change.test
  libFuzzer-x86_64-default-Windows :: fuzzer-implicit-signed-integer-truncation-or-sign-change.test
  libFuzzer-x86_64-default-Windows :: fuzzer-implicit-signed-integer-truncation.test
  libFuzzer-x86_64-default-Windows :: fuzzer-implicit-unsigned-integer-truncation.test
  libFuzzer-x86_64-default-Windows :: fuzzer-printcovpcs.test
  libFuzzer-x86_64-default-Windows :: fuzzer-timeout.test
  libFuzzer-x86_64-default-Windows :: fuzzer-ubsan.test
  libFuzzer-x86_64-default-Windows :: minimize_crash.test
  libFuzzer-x86_64-default-Windows :: minimize_two_crashes.test
  libFuzzer-x86_64-default-Windows :: null-deref-on-empty.test
  libFuzzer-x86_64-default-Windows :: null-deref.test
  libFuzzer-x86_64-default-Windows :: print-func.test
  libFuzzer-x86_64-default-Windows :: stack-overflow-with-asan.test
  libFuzzer-x86_64-default-Windows :: trace-malloc-2.test
  libFuzzer-x86_64-default-Windows :: trace-malloc-unbalanced.test
  libFuzzer-x86_64-default-Windows :: trace-malloc.test
```

The related commits are
53a81d4
and
e31efd8.
Following the change in
e31efd8
can fix these failures.

As for the issue mentioned in the comment that alternatename support in
clang not good enough(https://bugs.llvm.org/show_bug.cgi?id=40218). I
find that using `__builtin_function_start(func)` instead of directly
using `func` would make it work as intended.

Author: yingcong-wu

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWindows.cpp

@@ -22,6 +22,11 @@ using namespace fuzzer;
 #define STRINGIFY(A) STRINGIFY_(A)
 
 #if LIBFUZZER_MSVC
+#define GET_FUNCTION_ADDRESS(fn) &fn
+#else
+#define GET_FUNCTION_ADDRESS(fn) __builtin_function_start(fn)
+#endif // LIBFUZER_MSVC
+
 // Copied from compiler-rt/lib/sanitizer_common/sanitizer_win_defs.h
 #if defined(_M_IX86) || defined(__i386__)
 #define WIN_SYM_PREFIX "_"
@@ -31,17 +36,9 @@ using namespace fuzzer;
 
 // Declare external functions as having alternativenames, so that we can
 // determine if they are not defined.
-#define EXTERNAL_FUNC(Name, Default)                                   \
-  __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY( \
+#define EXTERNAL_FUNC(Name, Default)                                           \
+  __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY(         \
                                Name) "=" WIN_SYM_PREFIX STRINGIFY(Default)))
-#else
-// Declare external functions as weak to allow them to default to a specified
-// function if not defined explicitly. We must use weak symbols because clang's
-// support for alternatename is not 100%, see
-// https://bugs.llvm.org/show_bug.cgi?id=40218 for more details.
-#define EXTERNAL_FUNC(Name, Default) \
-  __attribute__((weak, alias(STRINGIFY(Default))))
-#endif  // LIBFUZZER_MSVC
 
 extern "C" {
 #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)         \
@@ -57,20 +54,23 @@ extern "C" {
 }
 
 template <typename T>
-static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
+static T *GetFnPtr(void *Fun, void *FunDef, const char *FnName,
+                   bool WarnIfMissing) {
   if (Fun == FunDef) {
     if (WarnIfMissing)
       Printf("WARNING: Failed to find function \"%s\".\n", FnName);
     return nullptr;
   }
-  return Fun;
+  return (T *)Fun;
 }
 
 namespace fuzzer {
 
 ExternalFunctions::ExternalFunctions() {
-#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN) \
-  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
+  this->NAME = GetFnPtr<decltype(::NAME)>(GET_FUNCTION_ADDRESS(::NAME),        \
+                                          GET_FUNCTION_ADDRESS(::NAME##Def),   \
+                                          #NAME, WARN);
 
 #include "FuzzerExtFunctions.def"