[gdb] Ensure listing of unused static var in info locals

Consider a test-case compiled with -g:
...
int main (void) {
  static int b = 2;
  return 0;
}
...

When running info locals in main, we get:
...
(gdb) info locals
No locals.
...

The info locals documentation states:
...
Print the local variables of the selected frame, each on a separate line.
These are all variables (declared either static or automatic) accessible at
the point of execution of the selected frame.
...
So, "info locals" should have printed static variable b.

The variable is present in dwarf info:
...
 <2><14a>: Abbrev Number: 6 (DW_TAG_variable)
    <14b>   DW_AT_name        : b
    <153>   DW_AT_const_value : 2
...
but instead of a location attribute, it has a const_value attribute, which
causes the corresponding symbol to have LOC_CONST, which causes info locals to
skip it.

Fix this by handling LOC_CONST in iterate_over_block_locals.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-02-24  Tom de Vries  <tdevries@suse.de>

	PR gdb/25592
	* stack.c (iterate_over_block_locals): Handle LOC_CONST.

gdb/testsuite/ChangeLog:

2020-02-24  Tom de Vries  <tdevries@suse.de>

	PR gdb/25592
	* gdb.base/info-locals-unused-static-var.c: New test.
	* gdb.base/info-locals-unused-static-var.exp: New file.

gdb/stack.c

@@ -2237,6 +2237,7 @@ iterate_over_block_locals (const struct block *b,
     {
       switch (SYMBOL_CLASS (sym))
 	{
+	case LOC_CONST:
 	case LOC_LOCAL:
 	case LOC_REGISTER:
 	case LOC_STATIC: