[clang-doc] Add html links to references

<a> tags are added for the parents and members of records and return type and
params of functions. The link redirects to the reference's info file.

The directory path where each info file will be saved is now generated in the
serialization phase and stored as an attribute in each Info.

Bitcode writer and reader were modified to handle the new attributes.

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

Differential Revision: https://reviews.llvm.org/D63663

llvm-svn: 365937