[msan] Stop propagating shadow in blacklisted functions.

With this change all values passed through blacklisted functions
become fully initialized. Previous behavior was to initialize all
loads in blacklisted functions, but apply normal shadow propagation
logic for all other operation.

This makes blacklist applicable in a wider range of situations.

It also makes code for blacklisted functions a lot shorter, which
works as yet another workaround for PR17409.