Add a new llvm::SmallVector template, which is similar to the vector class, but

Add a new llvm::SmallVector template, which is similar to the vector class, but
contains optimizations to avoid heap allocation if the vector size is smaller
than some threshold.  This can significantly improve the performance of code
that allocates many small vectors by eliminating tons of small malloc/free's.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29281 91177308-0d34-0410-b5e6-96231b3b80d8