Insert poisoned paddings between fields in C++ classes so that AddressSanitizer can find intra-object-overflow bugs

Summary:
The general approach is to add extra paddings after every field
in AST/RecordLayoutBuilder.cpp, then add code to CTORs/DTORs that poisons the paddings
(CodeGen/CGClass.cpp).

Everything is done under the flag -fsanitize-address-field-padding.
The blacklist file (-fsanitize-blacklist) allows to avoid the transformation
for given classes or source files.

See also https://code.google.com/p/address-sanitizer/wiki/IntraObjectOverflow

Test Plan: run SPEC2006 and some of the Chromium tests with  -fsanitize-address-field-padding

Reviewers: samsonov, rnk, rsmith

Reviewed By: rsmith

Subscribers: majnemer, cfe-commits

Differential Revision: http://reviews.llvm.org/D5687