[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:).

lib/SILOptimizer/Mandatory/PerformanceDiagnostics.cpp

@@ -519,6 +519,10 @@ bool PerformanceDiagnostics::visitInst(SILInstruction *inst,
         diagnose(loc, diag::embedded_swift_value_deinit, impactType.getASTType());
         return true;
       }
+      if (isa<KeyPathInst>(inst)) {
+        diagnose(loc, diag::embedded_swift_keypath);
+        return true;
+      }
       if (!allowedMetadataUseInEmbeddedSwift(inst)) {
         PrettyStackTracePerformanceDiagnostics stackTrace("metatype", inst);
         if (impactType) {