Simplify the computation of undefined symbols. Instead of walking

Simplify the computation of undefined symbols. Instead of walking
functions and initializers, just report the declarations present in
the module.

The motivation is to open the way for using the lazy module parsing,
which should speed up clients that just want a symbol list (nm, ar).

This is slightly less precise, but since both -strip-dead-prototypes
and -globaldce are part of the standard pipeline, this shouldn't
change the result for clang/dragonegg produced binaries.

Any decl in an IL file was also put there because a FE expected it
to be necessary, so this should not be a problem for "-O0 -emit-llvm".

As a sanity check, I have bootstrapped clang on linux and built
firefox on both linux and darwin. A clang bootstrap on darwin
with LTO fails with or without this patch because, ironically,
the linker doesn't like the combination of dead_strip and LTO
when building libLTO.so :-)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127870 91177308-0d34-0410-b5e6-96231b3b80d8