dwarf2read.c: Check type of string valued attributes prior to decoding.

dwarf2read.c: Check type of string valued attributes prior to decoding.

This change introduces a new function, dwarf2_string_attr(), which is
a wrapper for dwarf2_attr().  dwarf2read.c has been updated to
call dwarf2_string_attr in most instances where a string-valued
attribute is decoded to produce a string value.  In most cases, it
simplifies the code; in some instances, the complexity of the code
remains unchanged.

I performed this change by looking for instances where the
result of DW_STRING was used in an assignment.  Many of these
had a pattern which (roughly) looks something like this:

  struct attribute *attr = NULL;

  attr = dwarf2_attr (die, name, cu);
  if (attr != NULL && DW_STRING (attr))
    {
      const char *str;
      ...
      str = DW_STRING (attr);
      ... /* Use str in some fashion.  */
    }

Code of this form is transformed to look like this instead:

  const char *str;

  str = dwarf2_string_attr (die, name, cu)
  if (str != NULL)
    {
       ...
       /* Use str in some fashion.  */
       ...
    }

In addition to invoking dwarf2_attr() and DW_STRING(),
dwarf2_string_attr() checks to make sure that the attribute's
`form' field matches one of DW_FORM_strp, DW_FORM_string, or
DW_FORM_GNU_strp_alt.  If it does not match one of these forms,
it will return a NULL value in addition to calling complaint().

An earlier version of this patch did this type checking for one
particular instance where a string attribute was being decoded.
The situation that I was attempting to handle in that earlier patch is
this:

The Texas Instruments compiler uses the encoding for
DW_AT_MIPS_linkage_name for other purposes.  TI uses the encoding,
0x2007, for TI_AT_TI_end_line which, unlike DW_AT_MIPS_linkage_name,
does not have a string-typed value.  In this instance, GDB was attempting
to use an integer value as a string pointer, with predictable results.
(GDB would die with a segmentation fault.)

I've added a test which reproduces the problem that I was orignally
wanting to fix.  It uses DW_AT_MIPS_linkage name with an associate
value which is a string, and again, where the value is a small
integer.

My test case causes GDB to segfault in an unpatched GDB.  There
will be two PASSes in a patched GDB.

Unpatched GDB:

(gdb) ptype f
ERROR: Process no longer exists
UNRESOLVED: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype f
ERROR: Couldn't send ptype g to GDB.
UNRESOLVED: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype g

Patched GDB:

(gdb) ptype f
type = bool ()
(gdb) PASS: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype f
ptype g
type = bool ()
(gdb) PASS: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype g

I see no regressions on an x86_64 native target.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_string_attr): New function.
	(lookup_dwo_unit, process_psymtab_comp_unit_reader)
	(dwarf2_compute_name, dwarf2_physname, find_file_and_directory)
	(read_call_site_scope, namespace_name, guess_full_die_structure_name)
	(anonymous_struct_prefix, prepare_one_comp_unit): Use
	dwarf2_string_attr in place of dwarf2_attr and DW_STRING.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-bad-mips-linkage-name.c: New file.
	* gdb.dwarf2/dw2-bad-mips-linkage-name.exp: New file.