IR: only worry about threads in JvmIrSignatureDescriptor

General IdSignatureDescriptor is used from other backends, which have no
multi-threaded backend for now and do not need to carry the associated
runtime costs.

So IdSignatureDescriptor keeps the thread-unsafe caching of signature
builder.

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/signature/IdSignatureDescriptor.kt

@@ -105,15 +105,17 @@ open class IdSignatureDescriptor(private val mangler: KotlinMangler.DescriptorMa
             reportUnexpectedDescriptor(descriptor)
     }
 
+    private val composer by lazy { createSignatureBuilder() }
+
     override fun composeSignature(descriptor: DeclarationDescriptor): IdSignature? {
         return if (mangler.run { descriptor.isExported() }) {
-            createSignatureBuilder().buildSignature(descriptor)
+            composer.buildSignature(descriptor)
         } else null
     }
 
     override fun composeEnumEntrySignature(descriptor: ClassDescriptor): IdSignature? {
         return if (mangler.run { descriptor.isExportEnumEntry() }) {
-            createSignatureBuilder().buildSignature(descriptor)
+            composer.buildSignature(descriptor)
         } else null
     }
 }