[kotlin.reflect] Introduce ClassValue-based cache for KClassImpl

* Replace pcollections with ClassValue/ConcurrentHashMap-based caches
* Do not store weak references, instead cache strong references and count on ClassValue to unload the corresponding classloader if necessary
* ConcurrentHashMap does not rely on WeakReference as it's only selected on Android where classloader leaks don't exist
* Update reflect/scripting JDK requirement to Java 8 in order to proceed

#KT-53454
#KT-50705


Merge-request: KT-MR-6788
Merged-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

compiler/tests/org/jetbrains/kotlin/code/CodeConformanceTest.kt

@@ -31,7 +31,6 @@ class CodeConformanceTest : TestCase() {
                 "compiler/testData/psi/kdoc",
                 "compiler/tests/org/jetbrains/kotlin/code/CodeConformanceTest.kt",
                 "compiler/util/src/org/jetbrains/kotlin/config/MavenComparableVersion.java",
-                "core/reflection.jvm/src/kotlin/reflect/jvm/internal/pcollections",
                 "dependencies",
                 "dependencies/protobuf/protobuf-relocated/build",
                 "dist",