Be lazier when loading KeyFunctions from PCH/modules. We don't need to load

Be lazier when loading KeyFunctions from PCH/modules. We don't need to load
these in eagerly if we're not actually processing a translation unit. The added
laziness here also avoids us loading in parts of a CXXRecordDecl earlier than an
upcoming class template specialization merging patch would like.

Ideally, we should mark the vtable as used when we see a definition for the key
function, rather than having a separate pass over dynamic classes at the end of
the TU. The existing approach is pretty bad for PCH/modules, since it forcibly
loads the declarations of all key functions in all imported modules, whether or
not those key functions are defined.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189627 91177308-0d34-0410-b5e6-96231b3b80d8