[clang-doc] Fix bitcode writer for access specifiers

Bitcode writer was not emitting the corresponding record for the Access attribute of a FunctionInfo. This has been added.
AS_none was being used as the default value for any AcesssSpecifier attribute
(in FunctionInfo and MemberTypeInfo), this has been changed to AS_public
because this is the enum value that evaluates to 0.
The bitcode writer doesn't write values that are 0 so if an attribute
was set to AS_public, this value is not written and after reading the
bitcode it would have the default value which is AS_none. This is why
the default value is now AS_public.

Differential Revision: https://reviews.llvm.org/D66151

llvm-svn: 369063