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).

llvm-svn: 79237