Summary:
Describe the compile and runtime flags to enable MemorySanitizer
detection of use-after-destroy.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12914

Revise doc description of use-after-dtor.

Change wording to specify memory no longer readable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247871 91177308-0d34-0410-b5e6-96231b3b80d8