<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
Per discussion on the list. From now on, when you fix a compiler_crashers
test case, move it to compiler_crashers_fixed so that we get no-asserts and
ASan coverage.
Swift SVN r27055
