Register linkageSpecDecl matcher

Summary:
This allows `linkageSpecDecl` to be used from `clang-query`.

See also D31869 which similary adds `isStaticStorageClass`.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D37346

llvm-svn: 312283