pdbdump calls DbiStreamBuilder::commit through PDBFileBuilder::commit
without calling DbiStreamBuilder::finalize. Because `finalize` initializes
`Header` member, `Header` remained nullptr which caused a crash bug.
Differential Revision: https://reviews.llvm.org/D23143
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277681 91177308-0d34-0410-b5e6-96231b3b80d8