Mark test function as 'weak' to prevent interprocedural CSE.

A recent change to MemorySSA caused LLVM to start optimizing the call to
'f(x)' into just 'x', despite the 'noinline' attribute. So try harder to
prevent this optimization from firing.

compiler-rt/test/msan/no_sanitize_memory_prop.cpp

@@ -9,6 +9,7 @@
 #include <stdio.h>
 
 __attribute__((noinline))
+__attribute__((weak))
 __attribute__((no_sanitize_memory))
 int f(int x) {
   return x;