Replace some typically large vectors with SmallVector.

This may seem counter-intuitive but the POD-like optimization helps when the
vectors grow into multimegabyte buffers. SmallVector calls realloc which knows
how to twiddle virtual memory bits and avoids large copies.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175906 91177308-0d34-0410-b5e6-96231b3b80d8