[PM/ThinLTO] Fix PR33536, a bug where the ThinLTO bitcode writer was

querying for analysis results on a function declaration rather than
a definition.

The only reason this worked previously is by chance -- because the way
we got alias analysis results with the legacy PM, we happened to not
compute a dominator tree and so we happened to not hit an assert even
though it didn't make any real sense. Now we bail out before trying to
compute alias analysis so that we don't hit these asserts.

llvm-svn: 307625