Reinstate "Load lazily the template specialization in multi-module setups."
Reinstate "Load lazily the template specialization in multi-module setups."

It was reverted in r305460 but the issue appears to only break our self-host
libcxx modules bot. Reapplying it will give us a chance to get a reproducer and
fix the issue.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@306903 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang