[gdb/symtab] Skip local variables in cooked index

[gdb/symtab] Skip local variables in cooked index

Consider test-case gdb.dwarf2/local-var.exp.  The corresponding source
contains a function with a local variable:
...
program test
  logical :: local_var
  local_var = .TRUE.
end
...

Currently, the local variable shows up in the cooked index:
...
    [2] ((cooked_index_entry *) 0xfffec40063b0)
    name:       local_var
    canonical:  local_var
    qualified:  local_var
    DWARF tag:  DW_TAG_variable
    flags:      0x2 [IS_STATIC]
    DIE offset: 0xa3
    parent:     ((cooked_index_entry *) 0xfffec4006380) [test]
...
making the cooked index larger than necessary.

Fix this by skipping it in cooked_indexer::index_dies.

Tested on aarch64-linux.

PR symtab/32276
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32276