[clang-tidy] Ignore incomplete types when determining whether they are expensive to copy

Summary: IsExpensiveToCopy can return false positives for incomplete types, so ignore them.

All existing ClangTidy tests that depend on this function still pass as the types are complete.

Reviewers: alexfh, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26195

llvm-svn: 286008