fort_dyn_array: Fortran dynamic string support

This patch changes the semantic of the Dwarf string length
attribute to reflect the standard as well as enables
correct string length calculation of dynamic strings. Add
tests for varous dynamic string evaluations.

Old:
(gdb) p my_dyn_string
Cannot access memory at address 0x605fc0

New:
(gdb) p *my_dyn_string
$1 = 'foo'

gdb/Changlog:
	* dwarf2read.c (read_tag_string_type): changed
	semantic of DW_AT_string_length to be able to
	handle Dwarf blocks as well. Support for
	DW_AT_byte_length added to get correct length
	if specified in combination with
	DW_AT_string_length.
	(attr_to_dynamic_prop): added
	functionality to add Dwarf operators to baton
	data attribute. Added post values to baton
	as required by the string evaluation case.
	(read_subrange_type): Adapt caller.
	(set_die_type): Adapt caller.
	(add_post_values_to_baton): New function.
    	* gdbtypes.c (resolve_dynamic_type): Add
    	conditions to support string types.
    	(resolve_dynamic_array): Add conditions for dynamic
    	strings and create a new string type.
    	(is_dynamic_type): Follow pointer if a string type
    	was detected, as Fortran strings are represented
    	as pointers to strings internally.

gdb/testsuite/Changelog:
	* vla-strings.f90: New file.
	* vla-strings.exp: New file.

Change-Id: I7d7f47c7a4900a7fdb51102032455b53d60e60d7