BitcodeReader: Avoid std::vector with non-movable types from r267296

r267298 didn't quite fix the build errors.  Use SmallVector instead of
std::vector, the latter of which I think is trying to maintain a strong
exception safety guarantee.

http://lab.llvm.org:8011/builders/lldb-amd64-ninja-freebsd11/builds/6228

llvm-svn: 267299