[dsymutil] Gather global and local symbol addresses in the main executable.

Usually local symbols will have their address described in the debug
map. Global symbols have to have their address looked up in the symbol
table of the main executable. By playing with 'ld -r' and export lists,
you can get a symbol described as global by the debug map while actually
being a local symbol as far as the link in concerned. By gathering the
address of local symbols, we fix this issue.

Also, we prefer a global symbol in case of a name collision to preserve
the previous behavior.

Note that using the 'ld -r' tricks, people can actually cause symbol
names collisions that dsymutil has no way to figure out. This fixes the
simple case where there is only one symbol of a given name.

rdar://problem/32826621

Differential revision: https://reviews.llvm.org/D54922

llvm-svn: 348021