[LegacyLTO] Emit the error message that was silently dropped (llvm#152172)

Using LLVMContext to emit the error from `TargetRegistry::lookupTarget`
that was silently ignored and not propagated. This allows user to better
identify the kind of error occured.

rdar://157542119
(cherry picked from commit 09146a2)

Author: cachemeifyoucan

llvm/lib/LTO/LTOModule.cpp

@@ -204,8 +204,10 @@ LTOModule::makeLTOModule(MemoryBufferRef Buffer, const TargetOptions &options,
   // find machine architecture for this module
   std::string errMsg;
   const Target *march = TargetRegistry::lookupTarget(Triple, errMsg);
-  if (!march)
+  if (!march) {
+    Context.emitError(errMsg);
     return make_error_code(object::object_error::arch_not_found);
+  }
 
   // construct LTOModule, hand over ownership of module and target
   SubtargetFeatures Features;