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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219961 91177308-0d34-0410-b5e6-96231b3b80d8