Fix Rust parser bug with function fields

In Rust, 'obj.f()' is a method call -- but '(obj.f)()' is a call of a
function-valued field 'f' in 'obj'.  The Rust parser in gdb currently
gets this wrong.  This is PR rust/24082.

The expression and Rust parser rewrites made this simple to fix --
simply wrapping a parenthesized expression in a new operation handles
it.  This patch has a slight hack because I didn't want to introduce a
new exp_opcode enumeration constant just for this.  IMO this doesn't
matter, since we should work toward removing dependencies on these
opcodes anyway; but let me know what you think of this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24082