[asan] Be more careful and verbose when allocating dynamic shadow memory

FindAvailableMemoryRange can currently overwrite existing memory (by restricting the VM below addresses that are already used). This patch adds a check to make sure we don't restrict the VM space too much. We are also now more explicit about why the lookup failed and print out verbose values.

Differential Revision: https://reviews.llvm.org/D43318

llvm-svn: 326106