SI-8410 Don't warn fatally on disabled flag

Since boolean settings can now be set false by user,
summary warnings should not be issued when the flag
is explicitly off (as opposed to unset, default).

In particular, `-Xfatal-warnings` should not fail
if there were no warnings otherwise.

```
$ ~/scala-2.11.2/bin/scalac -d /tmp -deprecation:false test/files/pos/t8410.scala
$ ~/scala-2.11.2/bin/scalac -d /tmp -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala
warning: there were three deprecation warnings; re-run with -deprecation for details
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found

```
After this commit:
```
$ skalac -d /tmp -Xfatal-warnings test/files/pos/t8410.scala
warning: there were three deprecation warnings; re-run with -deprecation for details
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found
$ skalac -d /tmp -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala
```
Similarly for other collecting flags:
```
$ skalac -d /tmp -optimise -Yinline-warnings -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala
test/files/pos/t8410.scala:14: warning: Could not inline required method dropWhile because access level required by callee not matched by caller.
  def k = List(0).dropWhile(_ < 1)   // inlining warns doubly
                           ^
test/files/pos/t8410.scala:14: warning: At the end of the day, could not inline @inline-marked method dropWhile
  def k = List(0).dropWhile(_ < 1)   // inlining warns doubly
                           ^
error: No warnings can be incurred under -Xfatal-warnings.
two warnings found
one error found
$ skalac -d /tmp -optimise -Yinline-warnings:false -deprecation:false -Xfatal-warnings test/files/pos/t8410.scala

```
Footnote: handling of deprecated locals also changed in 2014:
```
$ ~/scala-2.11.0-M7/bin/scalac -d /tmp -deprecation -Xfatal-warnings test/files/pos/t8410.scala
test/files/pos/t8410.scala:8: warning: method f in object Test is deprecated:
  Console println f  // warns
                  ^
error: No warnings can be incurred under -Xfatal-warnings.
one warning found
one error found
$ ~/scala-2.11.0-M8/bin/scalac -d /tmp -deprecation -Xfatal-warnings test/files/pos/t8410.scala
test/files/pos/t8410.scala:5: warning: method _f is deprecated:
  def g = { @deprecated("","") def _f = f ; _f }                   // warns in 2.11.0-M8
                                            ^
test/files/pos/t8410.scala:6: warning: class X is deprecated:
  def x = { @deprecated("","") class X { def x = f } ; new X().x } // warns in 2.11.0-M8
                                                           ^
test/files/pos/t8410.scala:8: warning: method f in object Test is deprecated:
  Console println f  // warns
                  ^
error: No warnings can be incurred under -Xfatal-warnings.
three warnings found
one error found

```
I agree.  Another try at clarity: `warn` either reports immediately or buffers; `summarize` says tell me anything you haven't told me yet. No, wait.

OK, it seems to be the product of a massive merge:

https://github.com/scala/scala/commit/22d315d61b11e95c3a18e1285ca2131f614e13fb#diff-d686e8e67add4d42682f22193d20494b

Delta in the neighborhood:

https://github.com/scala/scala/commit/e0068b908517768e900a3945e483e9c379d728d8#diff-d686e8e67add4d42682f22193d20494b

I don't see anything on 2.10.x.  My assumption was that someone was putting something in the warnings map at some point and needed them flushed, but that was just a guess.  Everything has to go through the reporting front door now.  Unless @adriaanm says something different when he rises from the dead next Halloween.