[clang-doc] Redesign of generated HTML files

[clang-doc] Redesign of generated HTML files

The new design includes a header (contains the project name), a main section, and a footer.
The main section is divided into three subsections. Left, middle, right. The left section contains the general index, the middle contains the info's data, and the right contains the index for the info's content.
The CSS has been updated.
A flag --project-name is added.
The Attributes attribute of the TagNode struct is now a vector of pairs because these attributes should be rendered in the insertion order.
The functions (cpp and js) that converts an Index tree structure into HTML were slightly modified; the first ul tag created is now a ol tag. The inner lists are still ul.

Differential Revision: https://reviews.llvm.org/D66353

llvm-svn: 369139