If built-in operators could not be selected because of ambiguity in

user-defined type conversions, issue list of ambiguites in addition 
to the diagnostic. So, clang now issues the following:

b.cpp:19:19: error: left hand operand to ->* must be a pointer to class compatible with the right hand operand, but is 'struct C1'
        int i = c1->*pmf;
                ~~^
b.cpp:19:19: note: because of ambiguity in conversion of 'struct C1' to 'struct E *'
b.cpp:5:5: note: candidate function
    operator E*();
    ^
b.cpp:11:5: note: candidate function
    operator E*();
    ^

llvm-svn: 83862