swift234

Standardize terminology for "computed", "stored", "variable", and "property".

These are the terms sent out in the proposal last week and described in
StoredAndComputedVariables.rst.

variable
  anything declared with 'var'
member variable
  a variable inside a nominal type (may be an instance variable or not)
property
  another term for "member variable"
computed variable
  a variable with a custom getter or setter
stored variable
  a variable with backing storage; any non-computed variable

These terms pre-exist in SIL and IRGen, so I only attempted to solidify
their definitions. Other than the use of "field" for "tuple element",
none of these should be exposed to users.

field
  a tuple element, or
  the underlying storage for a stored variable in a struct or class
physical
  describes an entity whose value can be accessed directly
logical
  describes an entity whose value must be accessed through some accessor

Swift SVN r8698