SI-8219 Pending test case for unpositioned implicit error

Extracted with tweezers from ScalaTest and Scalacheck implicits.

    % scalac-hash v2.11.0-M7 test/pending/pos/t8219.scala
    error: type mismatch;
     found   : Any
     required: AnyRef
    Note: Any is not implicitly converted to AnyRef.  You can safely
    pattern match `x: AnyRef` or cast `x.asInstanceOf[AnyRef]` to do so.
    error: type mismatch;
     found   : Any
     required: AnyRef
    Note: Any is not implicitly converted to AnyRef.  You can safely
    pattern match `x: AnyRef` or cast `x.asInstanceOf[AnyRef]` to do so.
    two errors found
SI-8219 Avoid leaking ambiguous implicit errors from `viewExists`

This method is called during applicability checking, both to
determine if the given arguments are applicable to a candidate,
and to determine which overload to choose.

This was done using the current types Context. The call to
`inferImplicit` disabled immediate reporting of ambiguous implicit
errors, presumably as these should just be treated as a failed
implicit lookup.

But, in some cases, the buffered errors *were* later issued. Worse
still, they were issued at the position of the provided `EmptyTree`,
rather than at the call site being typechecked.

This commit uses silent mode to completely isolate the results of the
implicit search from the caller's Context.