Diagnose another case of the location counter moving backwards.

This case should be possible to handle, but it is hard:

* In order to create program headers correctly, we have to scan the
  sections in the order they are in the file.

* To find that order, we have to "execute" the linker script.

* The linker script can contain SIZEOF_HEADERS.

So to support this we have to start with a guess of how many headers
we need (3), run the linker script and try to create the program
headers. If it turns out we need more headers, we run the script again
with a larger SIZEOF_HEADERS.

Also, running the linker script depends on knowing the size of the
sections, so we have to finalize them. But creating the program
headers can change the value stored in some sections, so we have to
split size finalization and content finalization.

Looks like the last part is also needed for range extension thunks, so
we might support this at some point. For now just report an error
instead of producing broken files.

llvm-svn: 295458