gdb/disasm: fix demangling when disassembling the current function

When disassembling function symbols in C++ code, if GDB is asked to
disassemble a function by name then the function name in the header
line can be demangled by turning on `set print asm-demangle on`, e.g.:

  (gdb) disassemble foo_type::some_function
  Dump of assembler code for function _ZN8foo_type13some_functionE7my_type:
     0x0000000000401142 <+0>:	push   %rbp
     ... etc ...
  End of assembler dump.
  (gdb) set print asm-demangle on
  (gdb) disassemble foo_type::some_function
  Dump of assembler code for function foo_type::some_function(my_type):
     0x0000000000401142 <+0>:	push   %rbp
     ... etc ...                                                        │
  End of assembler dump.                                                │

However, if GDB is disassembling the current function, then this
demangling doesn't work, e.g.:

  (gdb) break foo_type::some_function
  Breakpoint 1 at 0x401152: file mangle.cc, line 16.
  (gdb) run
  Starting program: /tmp/mangle

  Breakpoint 1, foo_type::some_function (this=0x7fffffffa597, obj=...) at mangle.cc:16
  16	    obj.update ();
  (gdb) disassemble
  Dump of assembler code for function _ZN8foo_type13some_functionE7my_type:
     0x0000000000401142 <+0>:	push   %rbp
     ... etc ...
  End of assembler dump.
  (gdb) set print asm-demangle on
  (gdb) disassemble
  Dump of assembler code for function _ZN8foo_type13some_functionE7my_type:
     0x0000000000401142 <+0>:	push   %rbp
     ... etc ...
  End of assembler dump.

This commit fixes this issue, and extends gdb.cp/disasm-func-name.exp,
which was already testing the first case (disassemble by name) to also
cover disassembling the current function.

Approved-By: Tom Tromey <tom@tromey.com>