gn build: Split no-RTTI flag into a config.

Some of the sanitizer runtime code needs to be built with RTTI;
this allows that code to opt in to RTTI.

Differential Revision: https://reviews.llvm.org/D56627

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351155 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed