[asan] Move AsanCheckDynamicRTPrereqs check under flag

The patch addresses https://github.com/google/sanitizers/issues/786. Currently AsanCheckDynamicRTPrereqs prevents
dynamic ASan runtime from running in some important environments e.g. cowbuilder and fakeroot that may also work with interposition.
Let's allow users to switch off the check given that they know what they do.

Differential Revision: https://reviews.llvm.org/D31420

llvm-svn: 299188