SI-7253: respect binary compatibility constraints

From the JLS one can prove that moving a method to a superclass is a binary
compatible change, both forward and backward. That's because when compiling a
method call `c.foo()`, where c: C, the output descriptor *must* refer to `C` and
not to the class where `foo()` is actually defined.

This patch just ensures that, and adds a test comparing generated descriptors
against the Javac output.

The sample code is from Paul Philipps, the fix and the bytecode comparison code
from me.

From 2006 (9954eafffd5e60676238369ab0ed5797c92b4a7b, a fix for bug 455 in the
old bug tracker
http://www.scala-lang.org/sites/default/files/aladdin/displayItem.do%3Fid=455.html)
until 2.9, Scalac has followed this rule "often" (that is, when C is *not* an
interface).

This behavior was wrong, but the bug was hard to trigger. AFAICS, this can
create problems only when moving a method to a super interface in a library and
expecting forward binary compatibility - that is, compiling some Scala client
code against the new version of the library, and trying to run this code against
the old version of the library. This change grows an interface, so it is valid
only if clients are supposed to *not* implement the library.  Apparently, this
is so rare that nobody noticed.

Since 2.10 (0bea2ab5f6b211a83bbf14ea46fe57b8163c6334), Scalac follows this rule
*only* when C is an interface (I assume by oversight, since the main change was
an accessibility check), so the bug was finally triggered.

The new code will have to emit INVOKEINTERFACE instead of INVOKEVIRTUAL a bit
more often, compared to 2.9 (but not to 2.10). I don't know whether
INVOKEINTERFACE is noticeably slower (it shouldn't be); but this is the safest
fix since this behavior is mandated by the JLS.
If somebody disagrees and believes the 2.9 is significantly faster, IMHO he
should send a separate pull request (although ProGuard is probably a better
place for the change).
Remove fragile code, made redundant by previous commit

The JLS requires, as an exception to generate calls referring to Object for all
methods defined in Object itself; hence, whenever the method owner is Object,
the call should refer to the method owner. The previous commit implemented this
directly in useMethodOwner, so the special case for Object in isInterfaceCall is
now redudant. Hence, remove it.
SI-7253: update comments and naming

Rename isInterfaceCall -> needsInterfaceCall, which is more accurate since a
receiver can't literally be an interface call.
Merge pull request #2264 from Blaisorblade/issue/7253

SI-7253: respect binary compatibility constraints