[PDB] Serialize records into a stack-allocated buffer.

We were using a std::vector<> and resizing to MaxRecordLength,
which is ~64KB.  We would then do this repeatedly often many
times in a tight loop, which was causing measurable performance
impact when linking PDBs.

Patch by Alex Telishev
Differential Revision: https://reviews.llvm.org/D36940

llvm-svn: 311375