[PATCH] [gdb] Fix hard-coded constants in buildsym_compunit::make_blockvector

I came across some code in buildsym_compunit::make_blockvector that uses
hardcoded constants 0 and 1:
...
      gdb_assert (blockvector->block (0)->is_global_block ());
      gdb_assert (blockvector->block (1)->is_static_block ());
...

Fix this by instead using the symbolic constants GLOBAL_BLOCK and
STATIC_BLOCK.

The same function has an odd-looking for loop that uses a hard-coded '1' to
skip the global block:
...
       /* The 'J > 1' here is so that we don't place the global block into
 	 the map.  For CU with gaps, the static block will reflect the
 	 gaps, while the global block will just reflect the full extent of
 	 the range.  */
      for (int j = num_blocks; j > 1; )
 	{
	  --j;
 	  struct block *b = blockvector->block (j);
...

Fix this by rewriting it into an ordinary descending for loop, and using
symbolic constant GLOBAL_BLOCK to avoid the global block:
...
      for (int j = num_blocks - 1; j > GLOBAL_BLOCK; --j)
 	{
 	  struct block *b = blockvector->block (j);
...
---
 gdb/buildsym.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/gdb/buildsym.c b/gdb/buildsym.c
index b543cd10eb5..2b0a9a39b81 100644
--- a/gdb/buildsym.c
+++ b/gdb/buildsym.c
@@ -363,16 +363,15 @@ buildsym_compunit::make_blockvector ()
       gdb_assert (num_blocks > 1);
 
       /* Assert our understanding of how the blocks are laid out.  */
-      gdb_assert (blockvector->block (0)->is_global_block ());
-      gdb_assert (blockvector->block (1)->is_static_block ());
+      gdb_assert (blockvector->block (GLOBAL_BLOCK)->is_global_block ());
+      gdb_assert (blockvector->block (STATIC_BLOCK)->is_static_block ());
 
-      /* The 'J > 1' here is so that we don't place the global block into
-	 the map.  For CU with gaps, the static block will reflect the
-	 gaps, while the global block will just reflect the full extent of
+      /* The 'J > GLOBAL_BLOCK' here is so that we don't place the global
+	 block into the map.  For CU with gaps, the static block will reflect
+	 the gaps, while the global block will just reflect the full extent of
 	 the range.  */
-      for (int j = num_blocks; j > 1; )
+      for (int j = num_blocks - 1; j > GLOBAL_BLOCK; --j)
 	{
-	  --j;
 	  struct block *b = blockvector->block (j);
 
 	  gdb_assert (!b->is_global_block ());

base-commit: a4481c8ff808e7237b99815d84b0d4c06ded6124
-- 
2.51.0