Partially revert r91626. Materializing extra functions to determine whether

they're available_externally broke VMKit, which was relying on the fact that
functions would only be materialized when they were first called.  We'll have
to wait for http://llvm.org/PR5737 to really fix this.

I also added a test for one of the F->isDeclaration() calls which wasn't
covered by anything else in the test suite.

llvm-svn: 91943