[llvm] Use llvm::append_range (NFC) (#136066)
This patch replaces:
llvm::copy(Src, std::back_inserter(Dst));
with:
llvm::append_range(Dst, Src);
for breavity.
One side benefit is that llvm::append_range eventually calls
llvm::SmallVector::reserve if Dst is of llvm::SmallVector.
