[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>

libraries/reflect/api/build.gradle

@@ -2,7 +2,7 @@ description = ''
 
 apply plugin: 'kotlin'
 
-JvmToolchain.configureJvmToolchain(project, JdkMajorVersion.JDK_1_6)
+JvmToolchain.configureJvmToolchain(project, JdkMajorVersion.JDK_1_8)
 
 def includeJava9 = BuildPropertiesExtKt.getIncludeJava9(project.kotlinBuildProperties)
 

libraries/reflect/build.gradle.kts

@@ -20,7 +20,7 @@ plugins {
     `java-library`
 }
 
-configureJavaOnlyToolchain(JdkMajorVersion.JDK_1_6)
+configureJavaOnlyToolchain(JdkMajorVersion.JDK_1_8)
 
 publish()
 
@@ -146,7 +146,7 @@ val proguard by task<CacheableProguardTask> {
     injars(mapOf("filter" to "!META-INF/**,!**/*.kotlin_builtins"), proguardAdditionalInJars)
     outjars(fileFrom(base.libsDirectory.asFile.get(), "${base.archivesName.get()}-$version-proguard.jar"))
 
-    javaLauncher.set(project.getToolchainLauncherFor(chooseJdk18ForJpsBuild(JdkMajorVersion.JDK_1_6)))
+    javaLauncher.set(project.getToolchainLauncherFor(chooseJdk18ForJpsBuild(JdkMajorVersion.JDK_1_8)))
     libraryjars(mapOf("filter" to "!META-INF/versions/**"), proguardDeps)
     libraryjars(
         project.files(