swift234

Implement sane redeclaration checking.

Perform redeclaration checking of global and type-member declarations
at the time of declaration, using a notion of the signature of a
declaration to determine when one declaration is a redeclaration of
another. See ValueDecl::getOverloadSignature() and 
conflicting(OverloadSignature, OverloadSignature) for the specific
rules we implement. In a nutshell:

  - For functions, the signature includes:
    + The full name (which includes argument names)
    + The interface type
    + Whether the function is a class/static or instance method
    + For an operator, whether it is prefix or postfix
  - For a subscript, the signature is the interface type
  - For everything else, the signature is just the name

This tightens the rules in a number of ways, which is reflected in the
test case churn:

  - We now properly perform redeclaration checking for generics
  - We now propertly handle API argument names for functions
  - We now ban overloading between two variables of the same name but
    different type 
  - We now ban overloading between a variable/property and a function
  - We now ban overloading for initializers

The two test cases of actual interest are:

  test/decl/overload.swift: A bunch of new test cases for our checking

  test/Constraints/members.swift: I commented out a useful test for
  now, because it relies on overloading between a property and a
  function. We can reconsistute this test with a couple of modules.

This commit fixes at least a half dozen radars under the umbrella
<rdar://problem/11212777>. I still need to check them individually to
close them out.



Swift SVN r16691