[klib] Add a test ensuring that signature clash detection works w/ MPP

compiler/testData/diagnostics/klibSerializationTests/signatureClash_MPP.diag.txt

@@ -0,0 +1,16 @@
+Module: common
+/common.kt:7:1: error: Platform declaration clash: The following declarations have the same IR signature (/foo|foo(){}[0]):
+    fun foo(): kotlin.Int defined in root package
+    fun foo(): kotlin.String defined in root package
+
+/common.kt:10:1: error: Platform declaration clash: The following declarations have the same IR signature (/bar|bar(B){}[0]):
+    fun bar(x: B): kotlin.Int defined in root package
+    fun bar(x: B): kotlin.Int defined in root package
+Module: platform
+/platform.kt:14:1: error: Platform declaration clash: The following declarations have the same IR signature (/foo|foo(){}[0]):
+    fun foo(): kotlin.Int defined in root package
+    fun foo(): kotlin.String defined in root package
+
+/platform.kt:18:1: error: Platform declaration clash: The following declarations have the same IR signature (/bar|bar(B){}[0]):
+    fun bar(x: B): kotlin.Int defined in root package
+    fun bar(x: B): kotlin.Int defined in root package

compiler/testData/diagnostics/klibSerializationTests/signatureClash_MPP.kt

@@ -0,0 +1,18 @@
+// LANGUAGE: +MultiPlatformProjects
+// FIR_IDENTICAL
+// RENDER_ALL_DIAGNOSTICS_FULL_TEXT
+
+// MODULE: common
+// FILE: common.kt
+<!CONFLICTING_KLIB_SIGNATURES_ERROR!>fun foo(): String = ""<!>
+
+expect class A
+<!CONFLICTING_KLIB_SIGNATURES_ERROR!>fun bar(x: A): Int = 2<!>
+
+// MODULE: platform()()(common)
+// FILE: platform.kt
+<!CONFLICTING_KLIB_SIGNATURES_ERROR!>fun foo(): Int = 0<!>
+
+class B
+actual typealias A = B
+<!CONFLICTING_KLIB_SIGNATURES_ERROR!>fun bar(x: B): Int = 3<!>