irgen: introduce ManglePackagePath function

This is useful for clients that need to use llgo's mangling of the package
path to look up a specific function within a given package.

Differential Revision: http://reviews.llvm.org/D6801

llvm-svn: 225027
GitOrigin-RevId: 82a260eaaf36a51e962303ae637fe4aca67b52dd