[K/N] Tests for function attributes in bitcode generator

Besides, well, tests themselves, this commit extends FileCheck infra to
make it possible to test bitcode for direct and reverse interop.

kotlin-native/backend.native/tests/filecheck/signext_zeroext_interop_input.def

@@ -0,0 +1,20 @@
+---
+char char_id(char c) {
+    return c;
+}
+
+unsigned char unsigned_char_id(unsigned char c) {
+    return c;
+}
+
+short short_id(short s) {
+    return s;
+}
+
+unsigned short unsigned_short_id(unsigned short s) {
+    return s;
+}
+
+int callbackUser(int (*fn)(int, short)) {
+    return fn(5,5);
+}