Eval deprecation warning in @chiselRuntimeDeprecated

This ensures that Expressions that are not String literals will be
printed as String literals instead of Stringified ASTs. For example:

@chiselRuntimeDeprecated
@deprecated("This is " + "deprecated", "1.0")
def func() = ...

Formerly would print the runtime warning as:
"This is".$plus("deprecated")

It now prints the same as the Scala deprecation warning:
"This is deprecated"