Fix of default lookup for "this" symbol.

Fix of default lookup for "this" symbol.

Using the default lookup for the symbol "this" might lead to segmentation
fault in GDB.
Some languages, e.g. Fortran, use as default lookup routine the C++
routines.
For those languages "this" can be the instance of a class or even the
definition of a class.
When an instance of a class having the name "this" is evaluated
in GDB a segmentation fault was observed.

As example of the issue take into consideration the Fortran code:
  type foo
    real :: a
    type(bar) :: x
    character*7 :: b
  end type foo
  type(foo) :: this

Issue appears when evaluating the variable "this" in GDB.

Within the language definition structure there is a field that represents
the name of the special symbol used for the C++ "this" for the language
being described.
The fix presented here takes into account the aforementioned field. In the
case the aforementioned field is NULL "this" is not represented in the
language described and the lookup should return a null_block_symbol.

Tests: Performed tests with gfortran and ifort.

Reviewed:
https://sourceware.org/ml/gdb-patches/2016-04/msg00068.html

After the commited patch:
https://sourceware.org/ml/gdb-patches/2016-06/msg00364.html
Patch can be applied.

2016-06-16  Walfred Tedeschi  <walfred.tedeschi@intel.com>

gdb/ChangeLog:

	* cp-namespace.c (cp_lookup_bare_symbol): Use language passed as
	parameter to look for the symbol "this".

gdb/testsuite/ChangeLog:

	* gdb.fortran/derived-types.exp (result_line, result_line_2):
	New variables.
	(print this%a, print this%b, print this): New tests.
	* gdb.fortran/derived-types.f90 (this): New object and
	initialization.