[docs] Updated doxygen files to work well with doxygen 1.8.11

Doxygen 1.8.11 doesn't seem to like files with ".intro" extension by default.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262603 91177308-0d34-0410-b5e6-96231b3b80d8

docs/doxygen.cfg.in

@@ -745,7 +745,7 @@ WARN_LOGFILE           =
 
 INPUT                  = @abs_srcdir@/../include \
                          @abs_srcdir@/../lib \
-                         @abs_srcdir@/doxygen.intro
+                         @abs_srcdir@/doxygen-mainpage.dox
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses
@@ -1791,18 +1791,6 @@ GENERATE_XML           = NO
 
 XML_OUTPUT             = xml
 
-# The XML_SCHEMA tag can be used to specify a XML schema, which can be used by a
-# validating XML parser to check the syntax of the XML files.
-# This tag requires that the tag GENERATE_XML is set to YES.
-
-XML_SCHEMA             =
-
-# The XML_DTD tag can be used to specify a XML DTD, which can be used by a
-# validating XML parser to check the syntax of the XML files.
-# This tag requires that the tag GENERATE_XML is set to YES.
-
-XML_DTD                =
-
 # If the XML_PROGRAMLISTING tag is set to YES doxygen will dump the program
 # listings (including syntax highlighting and cross-referencing information) to
 # the XML output. Note that enabling this will significantly increase the size