[msan] Fix infinite recursion when _Unwind_Backtrace calls memcpy

[foad]

Summary

On PPC64, half the msan tests fail with an infinite recursion through
GetStackTrace like this:

#0 msan::GetStackTrace
#1 msan_memcpy
#2 ?? () from /lib64/libgcc_s.so.1
#3 ?? () from /lib64/libgcc_s.so.1
#4 _Unwind_Backtrace
#5 sanitizer::BufferedStackTrace::SlowUnwindStack
#6 sanitizer::BufferedStackTrace::Unwind
#7 msan::GetStackTrace
#8 interceptor_calloc
#9 _dl_allocate_tls
#10 pthread_create@@GLIBC_2.17
#11 __interceptor_pthread_create
#12 main

The problem is that we call _Unwind_Backtrace to get a stack trace; but
_Unwind_Backtrace calls memcpy, which we intercept and try to get
another stack trace.

This patch fixes it in __msan_memcpy by skipping the stack trace if
IsInSymbolizer(). This works because GetStackTrace already creates a
SymbolizerScope to "block reports from our interceptors during
_Unwind_Backtrace".

[eugenis]

LGTM

lib/msan/msan_interceptors.cc

 
 void __msan_clear_and_unpoison(void *a, uptr size) {
   REAL(memset)(a, 0, size);
   SetShadow(a, size, 0);
 }
 
 void *__msan_memcpy(void *dest, const void *src, SIZE_T n) {
   if (!msan_inited) return internal_memcpy(dest, src, n);
-  if (msan_init_is_running) return REAL(memcpy)(dest, src, n);
+  if (msan_init_is_running || __msan::IsInSymbolizer())
+    return REAL(memcpy)(dest, src, n);
   ENSURE_MSAN_INITED();
   GET_STORE_STACK_TRACE;
   void *res = REAL(memcpy)(dest, src, n);
   CopyShadowAndOrigin(dest, src, n, &stack);
   return res;
 }
 
 void *__msan_memset(void *s, int c, SIZE_T n) {