ASan+operator new[]: Add an option for more thorough operator new[] cookie poisoning

Summary:
Right now clang is skipping array cookie poisoning for any operator
new[] which is not part of the set of replaceable global allocation
functions.

This commit adds a flag to tell clang to poison all operator new[]
cookies.

A previous review was poisoning all array cookies unconditionally, but
there is an edge case which would stop working under ASan (a custom
operator new[] saves whatever pointer it returned, and then accesses
it).

This newer revision adds a command line argument to toggle this feature.

Original revision: https://reviews.llvm.org/D41301
Compiler-rt test revision with an explanation of the edge case: https://reviews.llvm.org/D41664

Reviewers: rjmccall, kcc, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43013

llvm-svn: 324884