The SectCreateMaterializationUnit creates a LinkGraph with a single named
section containing a single named block whose content is given by a
MemoryBuffer. It is intended to support emulation of ld64's -sectcreate option.
Replaces an llvm::cast that assumed that all Binary instances were either
Archive or MachOUniversalBinary instances with a dyn_cast. The cast was
triggering an assert in StaticLibraryDefinitionGenerator::Load if that method
was given a path or MemoryBuffer containing a relocatable object file.
Switching to dyn_cast causes the operation to error out with a bad-format
error as expected.
Fixes rdar://119262300
