[docs] Update the doxygen configuration file.

[docs] Update the doxygen configuration file.

Update the doxygen configuration file and Makefile build rules
to provide better output (simply use the default stylesheet and template
from the Doxygen distribution.)

This CL has upgrade doxygen.cfg.in to Doxygen 1.8.6.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232064 91177308-0d34-0410-b5e6-96231b3b80d8