CollectorMetadata abstractly describes stack maps for a function.

It includes:

 - location and of each safe point in machine code (identified by a
   label)
 - location of each root within the stack frame (identified by an
   offset), including the metadata tag provided to llvm.gcroot in
   the user program
 - size of the stack frame (for collectors which want to cheat on
   stack crawling :)
 - and eventually will include liveness

It is to be populated by back-ends during code-generation.

CollectorModuleMetadata aggregates this information across the
entire module.

llvm-svn: 42418