Skip generating DWARF info for dead code (#3498)

When encountering a subprogram that is dead code (as indicated by the
dead code proposal
https://dwarfstd.org/ShowIssue.php?issue=200609.1), don't generate debug
output for the subprogram or any of its children.

tests/all/debug/testsuite/dead_code.c

@@ -0,0 +1,19 @@
+int bar(int a)
+{
+  int b[50];
+  b[0] = a;
+  b[29] = a;
+  return a;
+}
+
+int baz(int a);
+
+__attribute__((export_name("foo"))) int foo()
+{
+  return baz(10);
+}
+
+__attribute__((noinline)) int baz(int a)
+{
+  return a + 5;
+}