swift234

Grab-bag of materialization-related changes:
  - introduce the concept of qualifiers on l-value types
  - teach overload resolution and coercion how to drop explicitness
  - require explicitness on normal [byref] arguments
  - make 'this' [byref(implicit)]
  - special-case '&' as a unary operator in the parser to make it
    produce an expression which type-checks as turning implicit l-values
    into explicit ones.

Obvious missing pieces:
  - updating LangRef
  - we should really complain about ever trying to rvalue-convert an
    explicit l-value
  - maybe qualification should play a part in overload resolution
  - we should diagnose attempts to declare unary '&' operators
  - there's a test case in expressions.swift which suggests my logic is
    slightly off

But I am out of time, and these will have to wait.

Swift SVN r1119