Rename -Xno-use-ir -> -Xuse-old-backend, add Gradle option

As soon as JVM IR is enabled by default (in language version 1.5), use
the CLI argument `-Xuse-old-backend` or Gradle option `useOldBackend` to
switch to the old JVM backend.

compiler/testData/cli/jvm/extraHelp.out

@@ -86,7 +86,6 @@ where advanced options include:
   -Xno-receiver-assertions   Don't generate not-null assertion for extension receiver arguments of platform types
   -Xno-reset-jar-timestamps  Do not reset jar entry timestamps to a fixed date
   -Xno-unified-null-checks   Use pre-1.4 exception types in null checks instead of java.lang.NPE. See KT-22275 for more details
-  -Xno-use-ir                Do not use the IR backend. Useful for a custom-built compiler where IR backend is enabled by default
   -Xprofile=<profilerPath:command:outputDir>
                              Debug option: Run compiler with async profiler, save snapshots to outputDir, command is passed to async-profiler on start
                              You'll have to provide async-profiler.jar on classpath to use this
@@ -116,6 +115,7 @@ where advanced options include:
                              Suppress the "cannot access built-in declaration" error (useful with -no-stdlib)
   -Xuse-ir                   Use the IR backend
   -Xuse-javac                Use javac for Java source and class files analysis
+  -Xuse-old-backend          Use the old JVM backend
   -Xuse-old-class-files-reading Use old class files reading implementation. This may slow down the build and cause problems with Groovy interop.
                              Should be used in case of problems with the new implementation
   -Xuse-14-inline-classes-mangling-scheme