[BuildLibCalls/SimplifyLibCalls] Fix attributes on created CallInst instructions.

- When emitting libcalls, do not only pass the calling convention from the
  function prototype but also the attributes.

- Do not pass attributes from e.g. libc memcpy to llvm.memcpy.

Review: Reid Kleckner, Eli Friedman, Arthur Eubanks

Differential Revision: https://reviews.llvm.org/D103992