Introduce safety checkin for ConcurrentValue conformance.

Introduce checking of ConcurrentValue conformances:
- For structs, check that each stored property conforms to ConcurrentValue
- For enums, check that each associated value conforms to ConcurrentValue
- For classes, check that each stored property is immutable and conforms
  to ConcurrentValue

Because all of the stored properties / associated values need to be
visible for this check to work, limit ConcurrentValue conformances to
be in the same source file as the type definition.

This checking can be disabled by conforming to a new marker protocol,
UnsafeConcurrentValue, that refines ConcurrentValue.
UnsafeConcurrentValue otherwise his no specific meaning. This allows
both "I know what I'm doing" for types that manage concurrent access
themselves as well as enabling retroactive conformance, both of which
are fundamentally unsafe but also quite necessary.

The bulk of this change ended up being to the standard library, because
all conformances of standard library types to the ConcurrentValue
protocol needed to be sunk down into the standard library so they
would benefit from the checking above. There were numerous little
mistakes in the initial pass through the stsandard library types that
have now been corrected.

stdlib/public/core/StringUnicodeScalarView.swift

@@ -61,7 +61,7 @@ extension String {
   ///     }
   ///     // Prints "My favorite emoji is "
   @frozen
-  public struct UnicodeScalarView {
+  public struct UnicodeScalarView: ConcurrentValue {
     @usableFromInline
     internal var _guts: _StringGuts
 
@@ -169,7 +169,7 @@ extension String.UnicodeScalarView: BidirectionalCollection {
 
 extension String.UnicodeScalarView {
   @frozen
-  public struct Iterator: IteratorProtocol {
+  public struct Iterator: IteratorProtocol, ConcurrentValue {
     @usableFromInline
     internal var _guts: _StringGuts