gdb: Add $_cimag and $_creal internal functions

gdb: Add $_cimag and $_creal internal functions

Add two new internal functions $_cimag and $_creal that extract the
imaginary and real parts of a complex value.

These internal functions can take a complex value of any type 'float
complex', 'double complex', or 'long double complex' and return a
suitable floating point value 'float', 'double', or 'long double'.
So we can now do this:

    (gdb) p z1
    $1 = 1.5 + 4.5 * I
    (gdb) p $_cimag (z1)
    $4 = 4.5
    (gdb) p $_creal (z1)
    $4 = 1.5

The components of a complex value are not strictly named types in
DWARF, as the complex type is itself the base type.  However, once we
are able to extract the components it makes sense to be able to ask
what the type of these components is and get a sensible answer back,
rather than the error we would currently get.  Currently GDB says:

    (gdb) ptype z1
    type = complex double
    (gdb) p $_cimag (z1)
    $4 = 4.5
    (gdb) ptype $
    type = <invalid type code 9>

With the changes in dwarf2read.c, GDB now says:

    (gdb) ptype z1
    type = complex double
    (gdb) p $_cimag (z1)
    $4 = 4.5
    (gdb) ptype $
    type = double

Which seems to make more sense.

gdb/ChangeLog:

	* NEWS: Mention new internal functions.
	* dwarf2read.c (dwarf2_init_complex_target_type): New function.
	(read_base_type): Use dwarf2_init_complex_target_type.
	* value.c (creal_internal_fn): New function.
	(cimag_internal_fn): New function.
	(_initialize_values): Register new internal functions.

gdb/doc/ChangeLog:

	* gdb.texinfo (Convenience Funs): Document '$_creal' and
	'$_cimag'.

gdb/testsuite/ChangeLog:

	* gdb.base/complex-parts.c: New file.
	* gdb.base/complex-parts.exp: New file.