Driver: begin threading frontend support for SymbolRewriter

Driver: begin threading frontend support for SymbolRewriter

Allow blessed access to the symbol rewriter from the driver. Although the
symbol rewriter could be invoked through tools like opt and llc, it would not
accessible from the frontend. This allows us to read the rewrite map files in
the frontend rather than the backend and enable symbol rewriting for actually
performing the symbol interpositioning.

llvm-svn: 225504