swift234

Implement 3 prominent feature requests:
<rdar://problem/15975935> warning that you can use 'let' not 'var'
<rdar://problem/18876585> Compiler should warn me if I set a parameter as 'var' but never modify it
<rdar://problem/17224539> QoI: warn about unused variables

This uses a simple pass in MiscDiagnostics that walks the body of an
AbstractFunctionDecl.  This means that it doesn't warn about unused properties (etc),
but it captures a vast majority of the cases.

It also does not warn about unused parameters (as a policy decision) because it is too noisy,
there are a variety of other refinements that could be done as well, thoughts welcome.



Swift SVN r28412