[ASan] Introduce SymbolizerPrepareForSandboxing(), which is a no-op on every...

[ASan] Introduce SymbolizerPrepareForSandboxing(), which is a no-op on every platform except Linux (because we don't support sandboxing anywhere else yet)
On Linux we pre-cache the value of readlink("/proc/self/exe"), so that it can be later used when the sandbox has been turned on.

llvm-svn: 182579