RFR: JDK-8190452: javac should not add MethodParameters attributes to v51 and earlier class files

[Liam Miller-Cushon]

Please review a fix for JDK-8190452. The change causes javac to not
emit MethodParameters
attributes when targeting v51 class files.

The change implements the suggestion from this thread:
http://mail.openjdk.java.net/pipermail/compiler-dev/2018-January/011579.html

There are two related changes that may be worth considering:
* now that -parameters will be ignored when compiling with --release < 8,
should this combination of flags result in a warning if -Xlint:options is
enabled?
* since this wasn't fixed in JDK 9, there are v51 class files in the wild
that contain unexpected MethodParameters attributes. Should
-Xlint:classfile be relaxed to avoid warning on those?

bug: https://bugs.openjdk.java.net/browse/JDK-8190452
webrev: http://cr.openjdk.java.net/~cushon/8190452/webrev.00/
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