[PDB] Defer writing the build id until the rest of the PDB is written.
[PDB] Defer writing the build id until the rest of the PDB is written.

For now this is NFC, but this small refactor opens the door to
letting us embed a hash of the PDB in the build id field of the
PDB.

Differential Revision: https://reviews.llvm.org/D43913

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@326453 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld