[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/AST/DiagnosticsSIL.def

@@ -379,6 +379,8 @@ ERROR(embedded_swift_metatype_type,none,
       "cannot use metatype of type %0 in embedded Swift", (Type))
 ERROR(embedded_swift_metatype,none,
       "cannot use metatype in embedded Swift", ())
+ERROR(embedded_swift_keypath,none,
+      "cannot use key path in embedded Swift", ())
 ERROR(embedded_swift_allocating_type,none,
       "cannot use allocating type %0 in -no-allocations mode", (Type))
 ERROR(embedded_swift_allocating,none,