Create an OutputSection for each non-empty OutputSectionCommand.

We were already pretty close, the one exception was when a name was
reused in another SECTIONS directive:

SECTIONS {
  .text : { *(.text) }
  .data : { *(.data) }
}
SECTIONS {
  .data : { *(other) }
}

In this case we would create a single .data and magically output
"other" while looking at the first OutputSectionCommand.

We now create two .data sections. This matches what gold does. If we
really want to create a single one, we should change the parser so that
the above is parsed as if the user had written

SECTIONS {
  .text : { *(.text) }
  .data : { *(.data) *(other)}
}

That is, there should be only one OutputSectionCommand for .data and
it would have two InputSectionDescriptions.

By itself this patch makes the code a bit more complicated, but is an
important step in allowing assignAddresses to operate just on the
linker script.

llvm-svn: 301484