The expr '.' is not absolute.

With this patch we keep track of the fact that . is a position in the
file and therefore not absolute. This allow us to compute relative
relocations that involve symbol that are defined in linker scripts
with '.'.

This fixes https://llvm.org/bugs/show_bug.cgi?id=30406

There is still more work to track absoluteness over the various
expressions, but this should unblock linking the EFI bootloader.

llvm-svn: 285641