Support magic literals as enum case raw value

Currently, the use of a magic literal (such as #file) as a raw value for an enum case:

enum Foo: String {
  case bar = #file
}

triggers an error: use of '#file' literal as raw value for enum case is not supported.

We should support use of a limited set of magic literals as raw values. The eligible literals would be #file, #filePath, #function, #line and #column since their raw value types can be used as the raw type for an enum:

enum FooString: String {
  case bar = #file // okay, #file is String which matches the raw type
}

enum FooInt: Int {
  case bar = #line // okay, #line is Int which matches the raw type
}

#line and #column can be easily fetched from the SourceManager, and the same with #file and #filePath I believe. #function name can be fetched by moving getMagicFunctionName from SILGen into AST (as a method on DeclContext).