DwarfDebug/DwarfUnit: Push abbreviation structures down into DwarfUnits to reduce duplication
[sanitizer] support toolchains that don't understand CFI directives

Summary: Support toolchains that don't understand CFI directives.

Reviewers: dvyukov

Reviewed By: dvyukov

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2336