Don't access the first element of a potentially empty

Don't access the first element of a potentially empty
vector (&Formals[0]).  With this change llvm-gcc builds
with expensive checking enabled for C, C++ and Fortran.
While there, change a std::vector into a SmallVector.
This is partly gratuitous, but mostly because not all
STL vector implementations define the data method (and
it should be faster).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79237 91177308-0d34-0410-b5e6-96231b3b80d8