[clang-doc] Fix typedef/using output.

Provides an initializer for the TypedefInfo.IsUsing member. Previously
this member was uninitialized and would produce random output.

Adds the Description (code comments) to the bitcode reader/writer.
Previously the typedef/using descriptions were lost during the bitcode
round-trip. Adds a test for this.

Differential Revision: https://reviews.llvm.org/D136638