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

test/decl/func/trailing_closures.swift

@@ -5,7 +5,7 @@
 func f1(f: () -> (), bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
 func f2(f: (() -> ())!, bar: Int = 10, wibble: Int = 17) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
 func f3(f: (() -> ())?, bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
-func f4(var f: (() -> ())?, bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
+func f4(var f: (() -> ())?, bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}} expected-warning {{parameter 'f' was never mutated}}
 
 // An extra set of parentheses suppresses the warning.
 func g1(f: (() -> ()), bar: Int = 10) { } // no-warning