[XRay][compiler-rt] Return the pointer associated with the function instead of the sled

[dberris]

Summary

XRay has erroneously been returning the address of the first sled in the
instrumentation map for a function id instead of the (runtime-relocated)
functison address. This causes confusion and issues for applications
where:

• The first sled in the function may not be an entry sled (due to re-ordering or some other reason).
• The caller attempts to find a symbol associated with the pointer at runtime, because the sled may not be exactly where the function's known address is (in case of inlined functions or those that have an external definition for symbols).

This fixes http://llvm.org/PR34340.

lib/xray/xray_interface.cc

 }
 
 int __xray_remove_handler_arg1() { return __xray_set_handler_arg1(nullptr); }
 
 uintptr_t __xray_function_address(int32_t FuncId) XRAY_NEVER_INSTRUMENT {
   __sanitizer::SpinMutexLock Guard(&XRayInstrMapMutex);
   if (FuncId <= 0 || static_cast<size_t>(FuncId) > XRayInstrMap.Functions)
     return 0;
-  return XRayInstrMap.SledsIndex[FuncId - 1].Begin->Address
+  return XRayInstrMap.SledsIndex[FuncId - 1].Begin->Function
 // On PPC, function entries are always aligned to 16 bytes. The beginning of a
 // sled might be a local entry, which is always +8 based on the global entry.
 // Always return the global entry.
 #ifdef __PPC__
          & ~0xf
 #endif
       ;
 }
 
 size_t __xray_max_function_id() XRAY_NEVER_INSTRUMENT {
   __sanitizer::SpinMutexLock Guard(&XRayInstrMapMutex);
   return XRayInstrMap.Functions;
 }