Serialize source range info for TypeTraitExpr.

This caused some crazy crashes involving std::unordered_map being
deserialized from a PCH file and then template instantiation requiring
an explicit instantiation location; unfortunately I don't really know
how to come up with a minimal test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197764 91177308-0d34-0410-b5e6-96231b3b80d8