Fix a copy of a fixed length, possibly non-nul terminated, string

into a std::string so we don't run off the end of the array when
there is no nul byte in ProcessElfCore::parseLinuxNotes.  
Found with ASAN testing.

<rdar://problem/37134319> 

Differential revision: https://reviews.llvm.org/D42828

llvm-svn: 324156