[libc++abi] Fix test under ASAN and MSAN

[smeenai]

Summary

When we're running tests under ASAN or MSAN, they're compiled with -O1,
which enables tail call elimination. This causes backtrace_test to fail:
the compiler performs tail call elimination for call3_nothrow, but it
can't for call3_throw, leading to a mismatched frame count. Disable tail
call elimination (and inlining, just to be explicit) to avoid this.

Test Plan:
Configured locally with -DLLVM_USE_SANITIZER=Address and was able to
reproduce the test failure. The test passes with this change.

[compnerd]

I think it would be better to use assert(true) to avoid the TCO from kicking in.

[smeenai]

Using attributes instead of compile flags, per compnerd's suggestion

[compnerd]

SVN r279935

test/backtrace_test.pass.cpp

 extern "C" _Unwind_Reason_Code
 trace_function(struct _Unwind_Context* context, void* ntraced) {
   (*reinterpret_cast<size_t*>(ntraced))++;
   // We should never have a call stack this deep...
   assert(*reinterpret_cast<size_t*>(ntraced) < 20);
   return _URC_NO_REASON;
 }
 
+__attribute__ ((__noinline__))
 void call3_throw(size_t* ntraced) {
   try {
     _Unwind_Backtrace(trace_function, ntraced);
   } catch (...) {
     assert(false);
   }
 }
 
+__attribute__ ((__noinline__, __disable_tail_calls__))
 void call3_nothrow(size_t* ntraced) {
   _Unwind_Backtrace(trace_function, ntraced);
 }
 
+__attribute__ ((__noinline__, __disable_tail_calls__))
 void call2(size_t* ntraced, bool do_throw) {
   if (do_throw) {
     call3_throw(ntraced);
   } else {
     call3_nothrow(ntraced);
   }
 }
 
+__attribute__ ((__noinline__, __disable_tail_calls__))
 void call1(size_t* ntraced, bool do_throw) {
   call2(ntraced, do_throw);
 }
 
 int main() {
   size_t throw_ntraced = 0;
   size_t nothrow_ntraced = 0;