Do not classify C struct members as a filename

There is existing logic in C/C++ expression parsing to avoid classifying
names as a filename when they are a field on the this object. This
change extends this logic to also avoid classifying names after a
struct-op (-> or .) as a filename, which otherwise causes a syntax
error.

Thus, it is now possible in the file

    #include <map>
    struct D {
        void map();
    }
    D d;

to call

    (gdb) print d.map()

where previously this would have been a syntax error.

Tested on gdb.cp/*.exp

gdb/ChangeLog:

        * c-exp.y (lex_one_token, classify_name, yylex): Don't classify
        names after a structop as a filename

gdb/testsuite/ChangeLog:

        * gdb.cp/filename.cc, gdb.cp/filename.exp: Test that member
        functions with the same name as an include file are parsed
        correctly.