swift234

Implement the 'warn_unused_result' attribute.

@warn_unused_result can be attached to function declarations to
produce a warning if the function is called but its result is not
used. It has two optional parameters that can be placed in
parentheses:

  message="some message": a message to include with the warning.

  mutable_variant="somedecl": the name of the mutable variant of the
  method that should be suggested when the subject method is called on
  a mutable value.

The specific use we're implementing this for now is for the mutating
and in-place operations. For example:

  @warn_unused_result(mutable_variant="sortInPlace") func sort() -> [Generator.Element] { ... }
  mutating func sortInPlace() { ... }

Translate Clang's __attribute__((warn_unused_result)) into
@warn_unused_result.

Implements rdar://problem/18165189.

Swift SVN r28019