rr::Coroutines are similar to rr::Functions in that it builds a new
executable function, but Coroutines have the following differences:
  (1) Coroutines do not support Return() statements.
  (2) Coroutines support Yield() statements to suspend execution of the
      coroutine and pass a value up to the caller. Yield can be called multiple
      times in a single execution of a coroutine.
  (3) The template argument T to Coroutine<T> is a C-style function
      signature.
  (4) Coroutine::operator() returns a rr::Stream<T> instead of an
      rr::Routine.
  (5) operator() starts execution of the coroutine immediately.
  (6) operator() uses the Coroutine's template function signature to
      ensure the argument types match the generated function signature.

It is my personal opinion that (3), (5) and (6) provide a more convenient and
safer interface and this could be adopted by rr::Function (post immediate
milestone).

Coroutines expose 3 externally callable functions:
(1) 'coroutine_begin' - the main entry point of the coroutine.
    Allocates a coroutine stack frame, and executes up to the first call to
    Yield().
(2) 'coroutine_await' - the function to collect a yielded value and resume
    execution of the suspended coroutine.
(3) 'coroutine_destroy' - the function to destruct and release the coroutine
    stack frame.

As there are now three exposed functions for coroutines, rr::Routine::getEntry()
now takes a function index. Standard rr::Functions always pass 0.
rr::Nucleus::CoroutineEntries is an enumerator of these coroutine functions that
correspond to the function index passed to rr::Routine::getEntry().

This change also adds third_party/llvm-7.0/stubs/Stubs.cpp. This file holds stub
functions that are never called by LLVM, but are referenced by the linker.
Actually linking in the file that declares these functions will drag in more
referenced files, significantly slowing the build and potentially bloating the
final executable size.

Coroutines are not currently supported by Subzero.

Bug: b/131672705
Change-Id: I0b13fe38b84c8dc85f4678019bf8d8afa7d9c37a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30212Tested-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>