[CMake] Add PARENT_TARGET option to add_sanitizer_rt_symbols.

Summary: This is another step in a multi-step refactoring to move add_sanitizer_rt_symbols in the direction of other add_* functions in compiler-rt.

Reviewers: filcab, bogner, kubabrecka, zaks.anna, glider, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12409

llvm-svn: 246178