CodeGen: Don't completely mess-up optimized atomic libcalls

Summary:
We did a great job getting this wrong:
- We messed up which LLVM IR types to use for arguments and return values.
  The optimized libcalls use integer types for values.

  Clang attempted to use the IR type which corresponds to the value
  passed in instead of using an appropriately sized integer type.  This
  would result in violations of the ABI for, as an example, floating
  point types.
- We didn't bother recording the result of the atomic libcall in the
  destination memory.

Instead, call the functions with arguments matching the type of the
libcall prototype's parameters.

This fixes PR20780.

Differential Revision: http://reviews.llvm.org/D5098