[llvm-nm] Fix use-after-free for MachOUniversalBinaries

MachOObjectFile::getHostArch() returns a temporary, and getArchName
returns a StringRef pointing to a temporary std::string.

No tests since it doesn't trigger any errors except with the sanitizers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347230 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed