Make CallingConv::ID an alias of "unsigned".

Summary:
Make CallingConv::ID a plain unsigned instead of enum with a
fixed set of valus. LLVM IR allows arbitraty calling conventions (you are
free to write cc12345), and loading them as enum is an undefined
behavior. This was reported by UBSan.

Test Plan: llvm regression test suite

Reviewers: nicholas

Reviewed By: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5248