[ASan] Do not rely on malloc context in allocator reports.

Invoke a fatal stack trace unwinder when ASan prints allocator-relevant
error reports (double-free, alloc-dealloc-mismatch, invalid-free).
Thus we'll be able to print complete stack trace even if allocation/free
stacks are not stored (malloc_context_size=0).

Based on the patch by Yuri Gribov!

llvm-svn: 194579