[lldb][clang][modern-type-lookup] Use ASTImporterSharedState in ExternalASTMerger

Summary:
The ExternalASTMerger should use the ASTImporterSharedState. This allows it to
handle std::pair in LLDB (but the rest of libc++ is still work in progress).

Reviewers: martong, shafik, a.sidorin

Subscribers: rnkovacs, christof, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68140

llvm-svn: 373193