[nfc] rename or eliminate isPureMoveOnly APIs

I think from SIL's perspective, it should only worry about whether the
type is move-only. That includes MoveOnlyWrapped SILTypes and regular
types that cannot be copied.

Most of the code querying `SILType::isPureMoveOnly` is in SILGen, where
it's very likely that the original AST type is sitting around already.
In such cases, I think it's fine to ask the AST type if it is
noncopyable. The clarity of only asking the ASTType if it's noncopyable
is beneficial, I think.

include/swift/SIL/SILType.h

@@ -748,10 +748,6 @@ public:
   /// wrapped type.
   bool isMoveOnly() const;
 
-  /// Is this a type that is a first class move only type. This returns false
-  /// for a move only wrapped type.
-  bool isPureMoveOnly() const;
-
   /// Return true if this is a value type (struct/enum) that requires
   /// deinitialization beyond destruction of its members.
   bool isValueTypeWithDeinit() const;