Add ability to decompile multifile classes

A side effect of KotlinClassFileDecompiler#KotlinDecompiledFileViewProvider@factory
that multifile java classes like FilesKt__FilePathComponentsKt.class
becomes not visible with Kotlin plugin as file
jar://..../kotlin-stdlib.jar!/kotlin/io/FilesKt__FilePathComponentsKt.class
is considered as Kotlin internal compiled file and therefore no PsiFile
for this virtualFile

#KTIJ-912

Merge-request: KT-MR-8146
Merged-by: Vladimir Dolzhenko <Vladimir.Dolzhenko@jetbrains.com>

analysis/decompiled/decompiler-to-file-stubs/src/org/jetbrains/kotlin/analysis/decompiler/stub/file/ClsClassFinder.kt

@@ -29,8 +29,7 @@ object ClsClassFinder {
      * which should NOT be decompiled (and, as a result, shown under the library in the Project view, be searchable via Find class, etc.)
      */
     fun isKotlinInternalCompiledFile(file: VirtualFile, fileContent: ByteArray? = null): Boolean {
-        // Don't crash on invalid files (EA-97751)
-        if (!file.isValid || fileContent?.size == 0 || !file.exists()) {
+        if (!file.isValidAndExists(fileContent)) {
             return false
         }
 
@@ -65,4 +64,17 @@ object ClsClassFinder {
         return header.kind == KotlinClassHeader.Kind.SYNTHETIC_CLASS ||
                 header.kind == KotlinClassHeader.Kind.MULTIFILE_CLASS_PART
     }
+
+    fun isMultifileClassPartFile(file: VirtualFile, fileContent: ByteArray? = null): Boolean {
+        if (!file.isValidAndExists(fileContent)) {
+            return false
+        }
+        val clsKotlinBinaryClassCache = ClsKotlinBinaryClassCache.getInstance()
+        val headerData = clsKotlinBinaryClassCache.getKotlinBinaryClassHeaderData(file, fileContent)
+        return headerData?.kind == KotlinClassHeader.Kind.MULTIFILE_CLASS_PART
+    }
+
+    // Don't crash on invalid files (EA-97751)
+    private fun VirtualFile.isValidAndExists(fileContent: ByteArray? = null): Boolean =
+        this.isValid && fileContent?.size != 0 && this.exists()
 }

analysis/decompiled/decompiler-to-psi/src/org/jetbrains/kotlin/analysis/decompiler/psi/KotlinClassFileDecompiler.kt

@@ -10,6 +10,7 @@ import com.intellij.psi.PsiManager
 import com.intellij.psi.compiled.ClassFileDecompilers
 import org.jetbrains.kotlin.analysis.decompiler.psi.file.KtClsFile
 import org.jetbrains.kotlin.analysis.decompiler.stub.file.ClsClassFinder.isKotlinInternalCompiledFile
+import org.jetbrains.kotlin.analysis.decompiler.stub.file.ClsClassFinder.isMultifileClassPartFile
 import org.jetbrains.kotlin.analysis.decompiler.stub.file.ClsKotlinBinaryClassCache
 import org.jetbrains.kotlin.analysis.decompiler.stub.file.KotlinClsStubBuilder
 
@@ -24,10 +25,11 @@ class KotlinClassFileDecompiler : ClassFileDecompilers.Full() {
         return KotlinDecompiledFileViewProvider(manager, file, physical) factory@{ provider ->
             val virtualFile = provider.virtualFile
 
-            if (isKotlinInternalCompiledFile(virtualFile))
+            if (isKotlinInternalCompiledFile(virtualFile) && !isMultifileClassPartFile(virtualFile)) {
                 null
-            else
+            } else {
                 KtClsFile(provider)
+            }
         }
     }
 }

analysis/decompiled/decompiler-to-psi/src/org/jetbrains/kotlin/analysis/decompiler/psi/text/buildDecompiledTextForClassFile.kt

@@ -37,7 +37,7 @@ fun buildDecompiledTextForClassFile(
     )
 
     return when (classHeader.kind) {
-        KotlinClassHeader.Kind.FILE_FACADE ->
+        KotlinClassHeader.Kind.FILE_FACADE, KotlinClassHeader.Kind.MULTIFILE_CLASS_PART ->
             buildText(resolver.resolveDeclarationsInFacade(classId.asSingleFqName()))
         KotlinClassHeader.Kind.CLASS -> {
             buildText(listOfNotNull(resolver.resolveTopLevelClass(classId)))