Improvements to the ASTImporter to support LLDB top-level Clang expressions.

The testcase for this is in LLDB, adeed by r264662.

This patch adds support for a variety of new expression types to the AST
importer, mostly related to C++.  It also adds support for importing lambdas
correctly, and adds support for importing the attributes attached to any Decl.

Finally, the patch adds a new templated function to ASTNodeImporter that imports
arbitrary arrays of importable things into a bump-allocated array attached to
getToContext().  This is a pattern we see at many places in ASTNodeImporter;
rather than do it slightly differently at each point, this function does it one
way.

<rdar://problem/22864976>

llvm-svn: 264669