Change the warning message about optimization to be printed once

per Module instead of once per CompileUnit, and print the 
module name.  A module may have a mix of compile units built with
optimization and compile units built without optimization -- the
warning won't be printed until the user selects a stack frame of
a function that was built with optimization.  And as before, it
will only be printed once per module per debug session.

<rdar://problem/19281172> 

llvm-svn: 244281