Sync with upstream version 6.0.3

[simpleton]

Version 6.0 (Feb. 2018)

Added new option -if, to specify conditions for -keep options.
Added new option -addconfigurationdebugging, to instrument the code to get feedback about missing configuration at runtime.
Added new option -android to tune processing for Android.
Added support for references to matched wildcards in regular expressions in -keep options.
Added new options -assumenoexternalsideeffects, -assumenoescapingparameters, and -assumenoexternalreturnvalues, to express fine-grained assumptions for better code optimization.
Added backporting of Java 8 code.
Added backporting and support for Java 9 code.
Improved vertical class merging.
[PGD-654] Fixed processing of MethodParameters attributes with nameless parameters.
[PGD-662] Fixed obfuscation causing clashing private and default method names.
[PGD-684] Fixed obfuscation of extensions of functional interfaces that are implemented with closures.
[PGD-681] Fixed potential IllegalArgumentException in simplification of tail recursion.
[PGD-672] Fixed memory leak writing compressed zip entries.
[PGD-652] Fixed possible NullPointerException due to missing variable initialization.
[PGD-641] Fixed possible NullPointerException due to optimized enum types.
[PGD-630] Fixed possible NullPointerException in optimization step.
[PGD-637] Fixed simplification of enum types in invokedynamic calls.
Fixed merging of classes sometimes resulting in final methods being overridden.
Fixed simplification of enum types that are stored in arrays.
Fixed VerifyError triggered by merging classes with shrinking disabled.