[clang-tools-extra] Cleanup documentation routine

The following issues are resolved:

* Doxygen didn't generate documentation for a bunch of existing tools
due to the absence of their directories in the doxygen configuration
file. This patch adds all relevant directories to the appropriate list.

* clang-tools-extra/docs/Doxyfile seems to be unused and irrelevant,
doxygen.cfg.in is passed to the CMake's Doxygen invocation, hence
Doxyfile is removed.

The validity of proposed changes was manually checked by building
doxygen-clang-tools and making sure that clangd and other tools are
present in Doxygen-generated docs of clang-tools-extra.

Reviewers: ioeric

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47537

llvm-svn: 336257