Add support of internal arguments for language feature settings

Arguments are passed in form '-XXLanguage:+LanguageFeatureName' for enabling
LanguageFeature.LanguageFeatureName, and '-XXLanguage:-LanguageFeatureName'
for disabling.

Note that they do override other settings, including 'language-version'
or extra ('-X') args.

compiler/testData/cli/jvm/internalArgEnableLanguageFeature.out

@@ -0,0 +1,11 @@
+warning: ATTENTION!
+This build uses internal compiler arguments:
+
+-XXLanguage:+InlineClasses
+
+This mode is strictly prohibited for production use,
+as no stability/compatibility guarantees are given on
+compiler or generated code. Use it at your own risk!
+
+warning: following manually enabled features will force generation of pre-release binaries: InlineClasses
+OK