[WebAssembly] Add no-prototype attribute to prototype-less C functions

The WebAssembly backend in particular benefits from being
able to distinguish between varargs functions (...) and prototype-less
C functions.

Differential Revision: https://reviews.llvm.org/D48443

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335510 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed