Updated the AST importer to support importing
LinkageSpecDecls.  This is relevant when LLDB
wants to import Decls from non-C++ modules,
since many declarations are in extern "C"
blocks.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223860 91177308-0d34-0410-b5e6-96231b3b80d8