[clang][OpenMP] Fix mangling of linear parameters.

Summary:
The linear parameter token in the mangling function must be multiplied
by the pointee size in bytes when the parameter is a pointer.

Reviewers: ABataev, andwar, jdoerfert

Subscribers: yaxunl, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78965