FindGlobalNamed used to take 2.92s out of 8.39s running a profile build of

gccld -disable-opt on 252.eon.

This patch deletes it and replaces it with a map.  The proper fix for this
is to fix PR411, but this will do in the short term.

gccld on eon now takes 5.51s, which is 50% faster than before this patch. :)

llvm-svn: 15480