Move the "findUsedStructTypes" functionality outside of the Module class.

Move the "findUsedStructTypes" functionality outside of the Module class.

The "findUsedStructTypes" method is very expensive to run. It needs to be
optimized so that LTO can run faster. Splitting this method out of the Module
class will help this occur. For instance, it can keep a list of seen objects so
that it doesn't process them over and over again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161228 91177308-0d34-0410-b5e6-96231b3b80d8