[tests] Disable clang optimization on the infinite recursion function.

In the recent llvm upstream change, https://reviews.llvm.org/D148269,
clang becomes smarter and will remove the infinite recursion function.
Use the clang attribute __attribute__((optnone)) to disable optimization
for it.

Bug: chromium:1435016
Change-Id: I74e823bf64d0b03d81c0bda7a8338e2fa67033aa
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/4456156
Reviewed-by: Mark Mentovai <[email protected]>
Commit-Queue: Zequan Wu <[email protected]>

Author: ZequanWu

client/crashpad_client_linux_test.cc

@@ -225,6 +225,9 @@ void ValidateDump(const StartHandlerForSelfTestOptions& options,
 #if defined(COMPILER_GCC)
 __attribute__((noinline))
 #endif
+#if defined(__clang__)
+__attribute__((optnone))
+#endif
 int RecurseInfinitely(int* ptr) {
   int buf[1 << 20];
   return *ptr + RecurseInfinitely(buf);