Output S_SECTION symbols to the Linker module.

PDBs need to contain 1 module for each object file/compiland,
and a special one synthesized by the linker.  This one contains
a symbol record for each output section in the executable with
its address information.  This patch adds such symbols to the
linker module.  Note that we also are supposed to add an
S_COFFGROUP symbol for what appears to be each input section that
contributes to each output section, but it's not entirely clear
how to generate these yet, so I'm leaving that for a separate
patch.

llvm-svn: 310754