Make ObjectFile ownership of the MemoryBuffer optional.

This allows llvm-ar to mmap the input files only once.
Adds a getSemanticSpelling function to semantic attribute subclasses which have a meaningful semantic spelling. Adds a sibling function to parsed attribtues (via AttributeList) for getting the semantic spelling, if one were to exist. This can be used for cleaner code that deals directly with the semantic spellings (such as the MSInheritance attribute).