Constant members should not contribute to layout

[saagarjha]

Description
Members that are constant and can never be modified should get folded out and stored on a per-type basis.

Steps to reproduce

struct Foo1 {
	var foo: Int { 0 }
}

struct Foo2 {
	let foo = 0
}

print(MemoryLayout<Foo1>.size)
print(MemoryLayout<Foo2>.size)

Expected behavior

0
0

Environment

• Swift compiler version info
  swift-driver version: 1.87.2 Apple Swift version 5.9 (swiftlang-5.9.2.1.6 clang-1500.1.0.1.1)
  Target: arm64-apple-macosx14.0
• Xcode version info
  Xcode 15.1
  Build version 15C5028h
• Deployment target: macOS 14.0

[grynspan]

Presumably this would not apply to constant members with non-constant initializers such as let x = Int.random(...).