[gdb/symtab] Handle DW_AT_decl_file with form DW_FORM_implicit_const

With test-case gdb.cp/temargs.exp on target board \
unix/gdb:debug_flags=-gdwarf-5 I run into:
...
(gdb) info addr I^M
ERROR: GDB process no longer exists
GDB process exited with wait status 32286 exp19 0 0 CHILDKILLED SIGABRT SIGABRT
UNRESOLVED: gdb.cp/temargs.exp: test address of I in templ_m
...

This is a regression since commit 529908cbd0 "Remove DW_UNSND".

The problem is that this DW_AT_decl_file:
...
 <1><221>: Abbrev Number: 4 (DW_TAG_structure_type)
    <222>   DW_AT_name        : Base<double, 23, (& a_global), &S::f>
    <226>   DW_AT_byte_size   : 1
    <226>   DW_AT_decl_file   : 1
    <226>   DW_AT_decl_line   : 30
    <227>   DW_AT_sibling     : <0x299>
...
is not read by this code in new_symbol:
....
      attr = dwarf2_attr (die,
                          inlined_func ? DW_AT_call_file : DW_AT_decl_file,
                          cu);
      if (attr != nullptr && attr->form_is_unsigned ())
...
because DW_AT_decl_file has form DW_FORM_implicit_const:
...
   4      DW_TAG_structure_type    [has children]
    DW_AT_name         DW_FORM_strp
    DW_AT_byte_size    DW_FORM_implicit_const: 1
    DW_AT_decl_file    DW_FORM_implicit_const: 1
    DW_AT_decl_line    DW_FORM_data1
    DW_AT_sibling      DW_FORM_ref4
    DW_AT value: 0     DW_FORM value: 0
...
which is a signed LEB128, so attr->form_is_unsigned () returns false.

Fix this by introducing new functions is_nonnegative and as_nonnegative, and
use these instead of form_is_unsigned and as_unsigned.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-24  Tom de Vries  <tdevries@suse.de>

	PR symtab/27336
	* dwarf2/attribute.c (attribute::form_is_signed): New function
	factored out of ...
	* dwarf2/attribute.h (attribute::as_signed): ... here.
	(attribute::is_nonnegative, attribute::as_nonnegative): New function.
	(attribute::form_is_signed): Declare.
	* dwarf2/read.c (new_symbol): Use is_nonnegative and as_nonnegative
	for DW_AT_decl_file.

gdb/ChangeLog

@@ -1,3 +1,14 @@
+2021-02-24  Tom de Vries  <tdevries@suse.de>
+
+	PR symtab/27336
+	* dwarf2/attribute.c (attribute::form_is_signed): New function
+	factored out of ...
+	* dwarf2/attribute.h (attribute::as_signed): ... here.
+	(attribute::is_nonnegative, attribute::as_nonnegative): New function.
+	(attribute::form_is_signed): Declare.
+	* dwarf2/read.c (new_symbol): Use is_nonnegative and as_nonnegative
+	for DW_AT_decl_file.
+
 2021-02-24  Kevin Buettner  <kevinb@redhat.com>
 
 	* nat/aarch64-linux-hw-point.c: Add comment regarding include

gdb/dwarf2/attribute.c

@@ -189,6 +189,14 @@ attribute::form_is_unsigned () const
 
 /* See attribute.h.  */
 
+bool
+attribute::form_is_signed () const
+{
+  return form == DW_FORM_sdata || form == DW_FORM_implicit_const;
+}
+
+/* See attribute.h.  */
+
 bool
 attribute::form_requires_reprocessing () const
 {

gdb/dwarf2/attribute.h

@@ -70,7 +70,7 @@ struct attribute
      form.  */
   LONGEST as_signed () const
   {
-    gdb_assert (form == DW_FORM_sdata || form == DW_FORM_implicit_const);
+    gdb_assert (form_is_signed ());
     return u.snd;
   }
 
@@ -92,6 +92,28 @@ struct attribute
     return u.unsnd;
   }
 
+  /* Return true if the value is nonnegative.  Requires that that
+     reprocessing not be needed.  */
+  bool is_nonnegative () const
+  {
+    if (form_is_unsigned ())
+      return true;
+    if (form_is_signed ())
+      return as_signed () >= 0;
+    return false;
+  }
+
+  /* Return the nonnegative value.  Requires that that reprocessing not be
+     needed.  */
+  ULONGEST as_nonnegative () const
+  {
+    if (form_is_unsigned ())
+      return as_unsigned ();
+    if (form_is_signed ())
+      return (ULONGEST)as_signed ();
+    gdb_assert (false);
+  }
+
   /* Return non-zero if ATTR's value is a section offset --- classes
      lineptr, loclistptr, macptr or rangelistptr --- or zero, otherwise.
      You may use the as_unsigned method to retrieve such offsets.
@@ -147,6 +169,9 @@ struct attribute
   /* Check if the attribute's form is an unsigned integer form.  */
   bool form_is_unsigned () const;
 
+  /* Check if the attribute's form is a signed integer form.  */
+  bool form_is_signed () const;
+
   /* Check if the attribute's form is a form that requires
      "reprocessing".  */
   bool form_requires_reprocessing () const;

gdb/dwarf2/read.c

@@ -22225,10 +22225,10 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
       attr = dwarf2_attr (die,
 			  inlined_func ? DW_AT_call_file : DW_AT_decl_file,
 			  cu);
-      if (attr != nullptr && attr->form_is_unsigned ())
+      if (attr != nullptr && attr->is_nonnegative ())
 	{
 	  file_name_index file_index
-	    = (file_name_index) attr->as_unsigned ();
+	    = (file_name_index) attr->as_nonnegative ();
 	  struct file_entry *fe;
 
 	  if (cu->line_header != NULL)