[gdb/symtab] Fix infinite recursion in dwarf2_cu::get_builder()

With the test-case attached in PR26327, gdb aborts:
...
$ gdb -q -batch 447.dealII -ex "b main"
Aborted (core dumped)
...
when running out of stack due to infinite recursion:
...
 #8  0x00000000006aaba6 in dwarf2_cu::get_builder (this=0x35e4b40)
     at src/gdb/dwarf2/read.c:700
 #9  0x00000000006aaba6 in dwarf2_cu::get_builder (this=0x22ee2c0)
     at src/gdb/dwarf2/read.c:700
 #10 0x00000000006aaba6 in dwarf2_cu::get_builder (this=0x35e4b40)
     at src/gdb/dwarf2/read.c:700
 #11 0x00000000006aaba6 in dwarf2_cu::get_builder (this=0x22ee2c0)
     at src/gdb/dwarf2/read.c:700
...

We're recursing in this code in dwarf2_cu::get_builder():
...
     /* Otherwise, search ancestors for a valid builder.  */
     if (ancestor != nullptr)
       return ancestor->get_builder ();
...
due to the fact that the ancestor chain is a cycle.

Higher up in the call stack, we find some code that is responsible for
triggering this, in new_symbol:
...
       case DW_TAG_formal_parameter:
         {
           /* If we are inside a function, mark this as an argument.  If
              not, we might be looking at an argument to an inlined function
              when we do not have enough information to show inlined frames;
              pretend it's a local variable in that case so that the user can
              still see it.  */
           struct context_stack *curr
             = cu->get_builder ()->get_current_context_stack ();
           if (curr != nullptr && curr->name != nullptr)
             SYMBOL_IS_ARGUMENT (sym) = 1;
...

This is code that was added to support pre-4.1 gcc, to be able to show
arguments of inlined functions as locals, in the absense of sufficiently
correct debug information.

Removing this code (that is, doing SYMBOL_IS_ARGUMENT (sym) = 1
unconditially), fixes the crash.  The ancestor variable also seems to have
been added specifically to deal with fallout from this code, so remove that as
well.

Tested on x86_64-linux:
- openSUSE Leap 15.2 with gcc 7.5.0, and
- openSUSE Tumbleweed with gcc 10.3.0.

gdb/ChangeLog:

2021-05-07  Tom de Vries  <tdevries@suse.de>

	PR symtab/26327
	* dwarf2/read.c (struct dwarf2_cu): Remove ancestor.
	(dwarf2_cu::get_builder): Remove ancestor-related code.
	(new_symbol): Remove code supporting pre-4.1 gcc that show arguments
	of inlined functions as locals.
	(follow_die_offset, follow_die_sig_1): Remove setting of ancestor.

gdb/doc/ChangeLog:

2021-05-07  Tom de Vries  <tdevries@suse.de>

	PR symtab/26327
	* gdb.texinfo (Inline Functions): Update.

1 files changed, 2 insertions, 23 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 5796cf1..b2f48f1 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -684,10 +684,6 @@ public:
 
   struct partial_die_info *find_partial_die (sect_offset sect_off);
 
-  /* If this CU was inherited by another CU (via specification,
-     abstract_origin, etc), this is the ancestor CU.  */
-  dwarf2_cu *ancestor;
-
   /* Get the buildsym_compunit for this CU.  */
   buildsym_compunit *get_builder ()
   {
@@ -695,10 +691,6 @@ public:
     if (m_builder != nullptr)
       return m_builder.get ();
 
-    /* Otherwise, search ancestors for a valid builder.  */
-    if (ancestor != nullptr)
-      return ancestor->get_builder ();
-
     return nullptr;
   }
 };
@@ -22045,15 +22037,7 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 	  break;
 	case DW_TAG_formal_parameter:
 	  {
-	    /* If we are inside a function, mark this as an argument.  If
-	       not, we might be looking at an argument to an inlined function
-	       when we do not have enough information to show inlined frames;
-	       pretend it's a local variable in that case so that the user can
-	       still see it.  */
-	    struct context_stack *curr
-	      = cu->get_builder ()->get_current_context_stack ();
-	    if (curr != nullptr && curr->name != nullptr)
-	      SYMBOL_IS_ARGUMENT (sym) = 1;
+	    SYMBOL_IS_ARGUMENT (sym) = 1;
 	    attr = dwarf2_attr (die, DW_AT_location, cu);
 	    if (attr != nullptr)
 	      {
@@ -23389,9 +23373,6 @@ follow_die_offset (sect_offset sect_off, int offset_in_dwz,
   *ref_cu = target_cu;
   temp_die.sect_off = sect_off;
 
-  if (target_cu != cu)
-    target_cu->ancestor = cu;
-
   return (struct die_info *) htab_find_with_hash (target_cu->die_hash,
 						  &temp_die,
 						  to_underlying (sect_off));
@@ -23741,7 +23722,7 @@ follow_die_sig_1 (struct die_info *src_die, struct signatured_type *sig_type,
 		  struct dwarf2_cu **ref_cu)
 {
   struct die_info temp_die;
-  struct dwarf2_cu *sig_cu, *cu = *ref_cu;
+  struct dwarf2_cu *sig_cu;
   struct die_info *die;
   dwarf2_per_objfile *per_objfile = (*ref_cu)->per_objfile;
 
@@ -23777,8 +23758,6 @@ follow_die_sig_1 (struct die_info *src_die, struct signatured_type *sig_type,
 	}
 
       *ref_cu = sig_cu;
-      if (sig_cu != cu)
-	sig_cu->ancestor = cu;
 
       return die;
     }