rearrange MCContext ownership. Before LLVMTargetMachine created it

and passing off ownership to AsmPrinter.  Now MachineModuleInfo
creates it and owns it by value.  This allows us to use MCSymbols
more consistently throughout the rest of the code generator, and
simplifies a bit of code.  This also allows MachineFunction to 
keep an MCContext reference handy, and cleans up the TargetRegistry
interfaces for AsmPrinters.

llvm-svn: 98450