Remove support for pnaclcall attribute

Summary:
It was used for interoperability with PNaCl's calling conventions, but
it's no longer needed.

Also Remove NaCl*ABIInfo which just existed to delegate to either the portable
or native ABIInfo, and remove checkCallingConvention which was now a no-op
override.

Reviewers: jvoung

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D7206

llvm-svn: 227362