gdb: Dynamic string length support

gdb: Dynamic string length support

Add support for strings with dynamic length using the DWARF attribute
DW_AT_string_length.

Currently gFortran generates DWARF for some strings that make use of
DW_AT_string_length like this:

 <1><2cc>: Abbrev Number: 20 (DW_TAG_string_type)
    <2cd>   DW_AT_string_length: 5 byte block: 99 bd 1 0 0      (DW_OP_call4: <0x1bd>)
    <2d3>   DW_AT_byte_size   : 4
    <2d4>   DW_AT_sibling     : <0x2e2>

In this type entry the DW_AT_string_length attribute references a
second DW_TAG_formal_parameter that contains the string length.  The
DW_AT_byte_size indicates that the length is a 4-byte value.

This commit extends GDB's DWARF parsing for strings so that we can
create dynamic types as well as static types, based on the attribute
the DWARF contains.

I then extend the dynamic type resolution code in gdbtypes.c to add
support for resolving dynamic strings.

gdb/ChangeLog:

	* dwarf2read.c (read_tag_string_type): Read the fields required to
	make a dynamic string, and possibly create a dynamic range for the
	string.
	(attr_to_dynamic_prop): Setup is_reference based on the type of
	attribute being processed.
	* gdbtypes.c (is_dynamic_type_internal): Handle TYPE_CODE_STRING.
	(resolve_dynamic_array): Rename to...
	(resolve_dynamic_array_or_string): ...this, update header comment,
	and accept TYPE_CODE_STRING.
	(resolve_dynamic_type_internal): Handle TYPE_CODE_STRING.

gdb/testsuite/ChangeLog:

	* gdb.fortran/array-slices.exp: Add test for dynamic strings.

Change-Id: I03f2d181b26156f48f27a03c8a59f9bd4d71ac17