This change adds an AssemblerInvocation class, similar to the CompilerInvocation class. It can be used to invoke cc1as directly.
The project I'm working on wants to compile Clang and use it as a static library. For that to work, there must be a way to invoke the assembler programmatically, using the same arguments as you would otherwise pass to cc1as.
This patch is outdated, updating it is fairly trivial. I'd like to have some feedback on the idea first, though.