[index] Added a method indexTopLevelDecls to run indexing on a list of Decls.

Summary:
We need it in clangd for refactoring that replaces ASTUnit
with manual AST management.

Reviewers: akyrtzi, benlangmuir, arphaman, klimek

Reviewed By: arphaman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D35405

llvm-svn: 308016