[msan] intercept dlopen and clear shadow for it

Summary:
The loader does not call mmap() through the PLT because it has to
bootstrap the process before libc is present.  Hooking dlopen() isn't
enough either because the loader runs module initializers before
returning, and they could run arbitrary msan instrumented code.

If msandr is present, then we can intercept the mmaps from dlopen at the
syscall layer and clear the shadow there.  If msandr is missing, we
clear the shadow after dlopen() and hope any initializers are trivial.

Reviewers: eugenis

CC: kcc, llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D509