[Object] Search for architecures by name in MachOUniversalBinary::getObjectForArch()

The reason we need to search by name rather than by Triple::ArchType
is to handle subarchitecture correclty. There is no different ArchType
for the x86_64h architecture (it identifies itself as x86_64), or for
the various ARM subarches. The only way to get to the subarch slice
in an universal binary is to search by name.

This issue led to hard to debug and transient symbolication failures
in Asan tests (it mostly works, because the files are very similar).

This also affects the Profiling infrastucture as it is the other user
of that API.

Reviewers: samsonov, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10604

llvm-svn: 240339