[MSan] Enable use-after-dtor instrumentation by default.

Summary:
Enable the compile-time flag -fsanitize-memory-use-after-dtor by
default. Note that the run-time option MSAN_OPTIONS=poison_in_dtor=1
still needs to be enabled for destructors to be poisoned.

Reviewers: eugenis, vitalybuka, kcc

Reviewed By: eugenis, vitalybuka

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D37860

llvm-svn: 322221