swift234

Replace infix attributes with operator decl lookup

During name binding, associate func decls with operator decls. When parsing SequenceExprs, look up operator decls to determine associativity and precedence of infix operators. Remove the infix_left and infix_left attributes, and make the infix attribute a simple declared attribute [infix] with no precedence.

Operator decls are resolved as follows:

- If an operator is declared in the same module as the use, resolve to the declaration in the current module.
- Otherwise, import operator declarations from all imported modules. If more than one declaration is imported for the operator and they conflict, raise an ambiguity error. If they are equivalent, pick one arbitrarily.

This allows operator declarations within the current module to override imported declarations if desired or to disambiguate conflicting operator declarations.

I've updated the standard library and the tests. stdlib2 and some of the examples still need to be updated.

Swift SVN r4629