Add kotlin-native/tools/degrade -- a helper for K/N Gradle builds

Intended to be helpful in minimizing reproducers or investigating
Kotlin/Native-related problems.

It analyses kotlin-multiplatform Gradle plugin events,
and creates shell scripts that run executed Kotlin/Native tools
(compiler, cinterop) without Gradle.

kotlin-native/tools/degrade/README.md

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+# degrade
+
+`degrade` is a tool that analyses kotlin-multiplatform Gradle plugin events,
+and creates shell scripts that run executed Kotlin/Native tools (compiler, cinterop)
+without Gradle.
+
+Intended to be helpful in minimizing reproducers or investigating
+Kotlin/Native-related problems.
+
+Confirmed to work with Kotlin versions from 1.5.10 to 1.6.0-M1.
+Probably doesn't work on Windows.
+
+## Usage
+
+```
+degrade <your usual Gradle invocation>
+```
+
+for example, in a Gradle project directory
+```
+degrade ./gradlew build
+```
+
+The tool runs Gradle build, analyses certain events during the run,
+and generates shell scripts in the `degrade` directory at the project root.
+It emits a script per Kotlin/Native task, and also `rerun-all.sh` and
+`rerun-failed.sh`.