[embedded] Build an initial embedded Swift standard library

This isn't a "complete" port of the standard library for embedded Swift, but
something that should serve as a starting point for further iterations on the
stdlib.

- General CMake logic for building a library as ".swiftmodule only" (ONLY_SWIFTMODULE).
- CMake logic in stdlib/public/core/CMakeLists.txt to start building the embedded stdlib for a handful of hardcoded target triples.
- Lots of annotations throughout the standard library to make types, functions, protocols unavailable in embedded Swift (@_unavailableInEmbedded).
- Mainly this is about stdlib functionality that relies on existentials, type erasure, metatypes, reflection, string interpolations.
- We rely on function body removal of unavailable functions to eliminate the actual problematic SIL code (existentials).
- Many .swift files are not included in the compilation of embedded stdlib at all, to simplify the scope of the annotations.
- EmbeddedStubs.swift is used to stub out (as unavailable and fatalError'd) the missing functionality.

lib/Frontend/Frontend.cpp

@@ -1042,6 +1042,8 @@ bool CompilerInvocation::shouldImportSwiftONoneSupport() const {
     return false;
   if (getSILOptions().shouldOptimize())
     return false;
+  if (LangOpts.hasFeature(Feature::Embedded))
+    return false;
 
   // If we are not executing an action that has a dependency on
   // SwiftOnoneSupport, don't load it.