SI-6923 Context now buffers warnings as well as errors

First pull request!

https://issues.scala-lang.org/browse/SI-6923

I want this change in 2.10.x but what's the procedure for porting this to 2.11/master?

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Code that was silently typed would not report warnings, even if it
returned a successful result.

This appeared in the following code which didn't show warnings even
with -Ywarn-adapted-args:

    def foo(a: Any) = a; foo(1, 2)

While the following would show the expected warning:

    def foo[A](a: Any) = a; foo(1, 2)