Add UnsafeRawPointer type and API. (#3677)

* Add UnsafeRawPointer type and API.

As proposed in SE-0107:   UnsafeRawPointer.
https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md

The fundamental difference between Unsafe[Mutable]RawPointer and
Unsafe[Mutable]Pointer<Pointee> is simply that the former is used for "untyped"
memory access, and the later is used for "typed" memory access. Let's refer to
these as "raw pointers" and "typed pointers". Because operations on raw pointers
access untyped memory, the compiler cannot make assumptions about the underlying
type of memory and must be conservative. With operations on typed pointers, the
compiler may make strict assumptions about the type of the underlying memory,
which allows more aggressive optimization.

Memory can only be accessed by a typed pointer when it is currently
bound to the Pointee type. Memory can be bound to type `T` via:
- `UnsafePointer<T>.allocate(capacity: n)`
- `UnsafePointer<Pointee>.withMemoryRebound(to: T.self, capacity: n) {...}`
- `UnsafeMutableRawPointer.initializeMemory(as: T.self, at: i, count: n, to: x)`
- `UnsafeMutableRawPointer.initializeMemory(as: T.self, from: p, count: n)`
- `UnsafeMutableRawPointer.moveInitializeMemory(as: T.self, from: p, count: n)`
- `UnsafeMutableRawPointer.bindMemory(to: T.self, capacity: n)`

Mangle UnsafeRawPointer as predefined substitution 'Sv' for Swift void
pointer ([urp] are taken).

* UnsafeRawPointer minor improvements.

Incorporate Dmitri's feedback.

Properly use a _memmove helper.

Add load/storeBytes alignment precondition checks.

Reword comments.

Demangler tests.

* Fix name mangling test cases.

* Fix bind_memory specialization.

lib/AST/Mangle.cpp

@@ -1392,6 +1392,12 @@ bool Mangler::tryMangleStandardSubstitution(const NominalTypeDecl *decl) {
   } else if (name == "Float") {
     Buffer << "Sf";
     return true;
+  } else if (name == "UnsafeRawPointer") {
+    Buffer << "SV";
+    return true;
+  } else if (name == "UnsafeMutableRawPointer") {
+    Buffer << "Sv";
+    return true;
   } else if (name == "UnsafePointer") {
     Buffer << "SP";
     return true;