Fix output section offset and contents when linker script uses memory region and data commands.

Advance the memory region offset when handling a linker script data
command such as BYTE or LONG.  Failure to advance the offset results
in corrupted output with overlapping sections.

Update tests to check for this combination of both a) memory regions
and b) data commands.

Fixes https://bugs.llvm.org/show_bug.cgi?id=35565

Patch by Owen Shaw!

llvm-svn: 321418