Update the doxygen configuration file and the 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/cfe/trunk@232066 91177308-0d34-0410-b5e6-96231b3b80d8