[clang] Implement support for -defsym assembler option

Summary:
Adds support for -Wa,-defsym,abc=1 option.

Related llvm patch: https://reviews.llvm.org/D26214

Reviewers: rafael, t.p.northover, colinl, echristo, compnerd, rengolin

Subscribers: mehdi_amini

Tags: #clang-c

Differential Revision: https://reviews.llvm.org/D26213

llvm-svn: 288397