[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:).

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyApply.swift

@@ -17,6 +17,10 @@ extension ApplyInst : OnoneSimplifyable {
     if tryTransformThickToThinCallee(of: self, context) {
       return
     }
+    if context.tryOptimizeKeypath(apply: self) {
+      context.erase(instruction: self)
+      return
+    }
     _ = context.tryDevirtualize(apply: self, isMandatory: false)
   }
 }