[Sanitizer] Fix debug builds of sanitizer_stacktrace_test.cpp

[luismarques]

Summary

An implementation of __sanitizer::BufferedStackTrace::UnwindImpl is provided per sanitizer, but there isn't one for sanitizer-common. In non-optimized builds of the sanitizer-common tests that becomes a problem: the test sanitizer_stacktrace_test.cpp won't have a reference to that method optimized away, causing linking errors. This patch provides a dummy implementation, which fixes those builds.

[vitalybuka]

That's bad, but I don't see a better easy way to fix that.
Maybe you create some separate sanitizer_common_test_runtime lib.

LGTM as is

compiler-rt/lib/sanitizer_common/tests/sanitizer_stacktrace_test.cpp

182

to crash even with NDEBUG
UNIMPLEMENTED();

[luismarques]

Address review feedback.

[luismarques]

In D96956#2573300, @vitalybuka wrote:

That's bad, but I don't see a better easy way to fix that.
Maybe you create some separate sanitizer_common_test_runtime lib.

I also thought about that, and considered that option, but in the end I concluded it wasn't worth it.
I will land this now. Thanks.

compiler-rt/lib/sanitizer_common/tests/sanitizer_stacktrace_test.cpp

TEST(SlowUnwindTest, ShortStackTrace) {
   EXPECT_EQ(0U, stack.size);
   EXPECT_EQ(0U, stack.top_frame_bp);
   stack.Unwind(pc, bp, nullptr, false, /*max_depth=*/1);
   EXPECT_EQ(1U, stack.size);
   EXPECT_EQ(pc, stack.trace[0]);
   EXPECT_EQ(bp, stack.top_frame_bp);
 }
 
+// Dummy implementation. This should never be called, but is required to link
+// non-optimized builds of this test.
+void BufferedStackTrace::UnwindImpl(uptr pc, uptr bp, void *context,
+                                    bool request_fast, u32 max_depth) {
+  UNIMPLEMENTED();
+}
+
 }  // namespace __sanitizer