[tsan] Fix memcmp interceptor to correctly use COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED

The memcmp interceptor checks COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED only after it calls COMMON_INTERCEPTOR_ENTER, which causes an early process launch crash when running TSan in iOS simulator. Let's fix this by checking COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED as the very first thing in the interceptor.

Differential Revision: http://reviews.llvm.org/D15287

llvm-svn: 255019