[Object] Extend MachOUniversalBinary::getObjectForArch

Make the method MachOUniversalBinary::getObjectForArch return MachOUniversalBinary::ObjectForArch
and add helper methods MachOUniversalBinary::getMachOObjectForArch, MachOUniversalBinary::getArchiveForArch
for those who explicitly expect to get a MachOObjectFile or an Archive.

Differential revision: https://reviews.llvm.org/D67700

Test plan: make check-all

llvm-svn: 372278
diff --git a/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp b/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp
index a555937..8e1852b 100644
--- a/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp
+++ b/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp
@@ -401,7 +401,7 @@
       return I->second.get();
 
     Expected<std::unique_ptr<ObjectFile>> ObjOrErr =
-        UB->getObjectForArch(ArchName);
+        UB->getMachOObjectForArch(ArchName);
     if (!ObjOrErr) {
       ObjectForUBPathAndArch.emplace(std::make_pair(Path, ArchName),
                                      std::unique_ptr<ObjectFile>());