Add experimental clang/driver flag -fsanitize-address-field-padding=N

Summary:
This change adds an experimental flag -fsanitize-address-field-padding=N (0, 1, 2)
to clang and driver. With this flag ASAN will be able to detect some cases of
intra-object-overflow bugs,
see https://code.google.com/p/address-sanitizer/wiki/IntraObjectOverflow

There is no actual functionality here yet, just the flag parsing.
The functionality is being reviewed at http://reviews.llvm.org/D5687

Test Plan: Build and run SPEC, LLVM Bootstrap, Chrome with this flag.

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5676
Move DwarfUnit::constructVariableDIE down to DwarfCompileUnit, since it's only needed there.
Sink DwarfDebug.cpp::constructVariableDIE into DwarfCompileUnit.