Add basic support for multi-platform projects to the compiler (#1254)

Add compiler pass removing `expect` declarations

Thus support multi-platform projects in the compiler.

Also 

* Add some trivial checks to ensure that `expect` declarations
are not generated to bitcode.
* Add minor improvement to exported symbol name clash detection.
* Don't create LLVM declarations for Obj-C-interop fake functions.

backend.native/tests/build.gradle

@@ -1918,6 +1918,11 @@ task memory_escape2(type: RunKonanTest) {
     source = "runtime/memory/escape2.kt"
 }
 
+task mpp1(type: RunStandaloneKonanTest) {
+    source = "codegen/mpp/mpp1.kt"
+    flags = ['-tr', '-Xmulti-platform']
+}
+
 task unit1(type: RunKonanTest) {
     goldValue = "First\nkotlin.Unit\n"
     source = "codegen/basics/unit1.kt"

backend.native/tests/codegen/mpp/mpp1.kt

@@ -0,0 +1,31 @@
+package codegen.mpp.mpp1
+
+import kotlin.test.*
+
+fun box() {
+    assertEquals(A().B().fourtyTwo(), 42)
+    assertEquals(seventeen(), 17)
+}
+
+expect class A {
+    constructor()
+
+    inner class B {
+        fun fourtyTwo(): Int
+
+        constructor()
+    }
+}
+
+actual class A {
+    actual inner class B actual constructor() {
+        actual fun fourtyTwo() = 42
+    }
+}
+
+actual fun seventeen() = 17
+expect fun seventeen(): Int
+
+@Test fun runTest() {
+    box()
+}