[msan] Fix infinite recursion when _Unwind_Backtrace calls memcpy

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".

Reviewers: samsonov, wschmidt, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10762

llvm-svn: 240878