cmd/compile: set conversions to unsafe.Pointer as an escaping operation when -asan is enabled
When ASan is enabled, treat conversions to unsafe.Pointer as
an escaping operation. In this way, all pointer operations on
the stack objects will become operations on the escaped heap
objects. As we've already supported ASan detection of error
memory accesses to heap objects. With this trick, we can use
-asan option to report errors on bad stack operations.