[embedded] Compile-time (literal) KeyPaths for Embedded Swift

Enable KeyPath/AnyKeyPath/PartialKeyPath/WritableKeyPath in Embedded Swift, but
for compile-time use only:

- Add keypath optimizations into the mandatory optimizations pipeline
- Allow keypath optimizations to look through begin_borrow, to make them work
  even in OSSA.
- If a use of a KeyPath doesn't optimize away, diagnose in PerformanceDiagnostics
- Make UnsafePointer.pointer(to:) transparent to allow the keypath optimization
  to happen in the callers of UnsafePointer.pointer(to:).

include/swift/SILOptimizer/Utils/InstOptUtils.h

@@ -599,6 +599,13 @@ bool specializeAppliesInFunction(SILFunction &F,
                                  SILTransform *transform,
                                  bool isMandatory);
 
+bool tryOptimizeKeypath(ApplyInst *AI, SILBuilder Builder);
+bool tryOptimizeKeypathApplication(ApplyInst *AI, SILFunction *callee, SILBuilder Builder);
+bool tryOptimizeKeypathOffsetOf(ApplyInst *AI, FuncDecl *calleeFn,
+                                KeyPathInst *kp, SILBuilder Builder);
+bool tryOptimizeKeypathKVCString(ApplyInst *AI, FuncDecl *calleeFn,
+                                KeyPathInst *kp, SILBuilder Builder);
+
 /// Instantiate the specified type by recursively tupling and structing the
 /// unique instances of the empty types and undef "instances" of the non-empty
 /// types aggregated together at each level.