[dfsan] Replace dfs$ prefix with .dfsan suffix

The current naming scheme adds the `dfs$` prefix to all
DFSan-instrumented functions.  This breaks mangling and prevents stack
trace printers and other tools from automatically demangling function
names.

This new naming scheme is mangling-compatible, with the `.dfsan`
suffix being a vendor-specific suffix:
https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling-structure

With this fix, demangling utils would work out-of-the-box.

Reviewed By: stephan.yichao.zhao

Differential Revision: https://reviews.llvm.org/D104494