[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.

stdlib/public/core/CompilerProtocols.swift

@@ -762,6 +762,7 @@ public protocol ExpressibleByDictionaryLiteral {
 /// `StringInterpolationProtocol` and have a matching `StringLiteralType`.
 ///
 /// For more information, see the `StringInterpolationProtocol` documentation.
+@_unavailableInEmbedded
 public protocol ExpressibleByStringInterpolation
   : ExpressibleByStringLiteral {
   
@@ -787,6 +788,7 @@ public protocol ExpressibleByStringInterpolation
   init(stringInterpolation: StringInterpolation)
 }
 
+@_unavailableInEmbedded
 extension ExpressibleByStringInterpolation
   where StringInterpolation == DefaultStringInterpolation {
   
@@ -940,6 +942,7 @@ public protocol _ExpressibleByColorLiteral {
 
 /// A type that can be initialized using an image literal (e.g.
 /// `#imageLiteral(resourceName: "hi.png")`).
+@_unavailableInEmbedded
 public protocol _ExpressibleByImageLiteral {
   /// Creates an instance initialized with the given resource name.
   ///
@@ -950,6 +953,7 @@ public protocol _ExpressibleByImageLiteral {
 
 /// A type that can be initialized using a file reference literal (e.g.
 /// `#fileLiteral(resourceName: "resource.txt")`).
+@_unavailableInEmbedded
 public protocol _ExpressibleByFileReferenceLiteral {
   /// Creates an instance initialized with the given resource name.
   ///