Refactor generic Asm directive parsing.

After discussing the refactoring with Jim and Daniel, the following changes were
made:

* All generic directive parsing is now done by AsmParser itself. The previous
  division between it and GenericAsmParser did not have clear boundaries and
  just produced unnatural code of GenericAsmParser juggling the internals of
  AsmParser through an interface.
  The division of responsibilities is now clear: target-specific directives,
  other extensions (used by platform-specific parseres), and generic directives.
* Priority for directive parsing was reshuffled to ask extensions first and
  check the generic directives later.

No change in functionality.