[cfi] Test case for weak symbol handling

[eugenis]

Summary

CFI runtime test for D25854

This code used to crash because CFI emits a jump table entry for "does_not_exist" which itself is not weak (it can't be), but it jumps to nullptr.

test/cfi/icall/weak.c

+// Test that weak symbols stay weak.
+// RUN: %clang_cfi -lm -o %t1 %s && %t1
+
+__attribute__((weak)) void does_not_exist(void);
+
+__attribute__((noinline))
+void foo(void (*p)(void)) {
+  p();
+}
+
+int main(int argc, char **argv) {
+  if (does_not_exist)
+    foo(does_not_exist);
+}