[clang-doc] De-duplicate comments and locations

De-duplicate comments and declaration locations in reduce function.
When two files include the same header file, this file's content is mapped
twice causing comments and locations to be duplicated after the reduce stage.

Committed on behalf of Diego Astiazarán (diegoaat97@gmail.com).

Differential Revision: https://reviews.llvm.org/D62970

llvm-svn: 364670