[MC] MCDwarfLineAddr::Encode: use SmallVectorImpl instead of raw_ostream. NFC

Similar to D145791: most call sites need a SmallString, but have to provide a
raw_svector_ostream wrapper with unneeded abstraction and overhead:

raw_ostream::write =(inlinable)=> flush_tied_then_write (unneeded TiedStream check) =(virtual function call)=> raw_svector_ostream::write_impl ==> SmallVector append(ItTy in_start, ItTy in_end) (range; less efficient then push_back).

Just use SmallVectorImpl to simplify and optimize code. Unfortunately most call
sites use SmallString, so we have to use SmallVectorImpl<char> instead of
<uint8_t> to avoid large refactoring.