* [AST] Add 'FloatingPoint' known protocol kind
* [Sema] Emit a diagnostic for comparisons with '.nan' instead of using '.isNan'
* [Sema] Update '.nan' comparison diagnostic wording
* [Sema] Explicitly check for either arguments to be '.nan' and tweak a comment
* [Test] Tweak some comments
* [Diagnostic] Change 'isNan' to 'isNaN'
* [Sema] Fix a bug where firstArg was checked twice for FloatingPoint conformance and update some comments
* [Test] Fix comments in test file
* [NFC] Add a new 'isStandardComparisonOperator' method to 'Identifier' and use it in ConstraintSystem
* [NFC] Reuse argument decl extraction code and switch over to the new 'isStandardComparisonOperator' method
* [NFC] Update conformsToKnownProtocol to accept DeclContext and use it to check for FloatingPoint conformance
