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

include/swift/AST/DiagnosticsSema.def

@@ -2265,6 +2265,28 @@ ERROR(availability_declaration_less_available_than_protocol, sema_avail,
 NOTE(availability_protocol_requirement_here, sema_avail, none,
      "protocol requirement here", ())
 
+
+//------------------------------------------------------------------------------
+// Variable usage diagnostics
+//------------------------------------------------------------------------------
+
+WARNING(pbd_never_used, sema_varusage, none,
+        "initialization of %select{variable|immutable value}1 %0 was never used"
+        "; consider replacing with assignment to '_' or removing it",
+        (Identifier, unsigned))
+
+WARNING(variable_never_used, sema_varusage, none,
+        "%select{variable|immutable value}1 %0 was never used; "
+        "consider replacing with '_' or removing it",
+        (Identifier, unsigned))
+WARNING(variable_never_mutated, sema_varusage, none,
+        "%select{variable|parameter}1 %0 was never mutated; "
+        "consider changing to 'let' constant",
+        (Identifier, unsigned))
+WARNING(variable_never_read, sema_varusage, none,
+        "%select{variable|parameter}1 %0 was written to, but never read",
+        (Identifier, unsigned))
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG