[pdb] Introduce MsfBuilder for laying out PDB files.

Reviewed by: ruiu
Differential Revision: https://reviews.llvm.org/D22308

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275611 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed