Type-lookup by name for type in namespace fails with -fdebug-types-section after 93b8bea

Hi Tom,

We have found a strange error with -fdebug-types-section, and as best I can
tell, your commit 93b8bea4143cafae79076076c64aaa4c46a9b73c is the culprit,
although I don't understand how or why.

In any event, before this change, the test-case below works fine, and after
it, lookup of a type in a namespace by name fails. In particular, "ptype
hide::Bar" should find the type. But it fails both inside python with
gdb.lookup_type and with normal commands.

Any idea what is happening? Reproducer follows.

$ g++ -v
...
gcc version 9.2.1 20191102 (Debian 9.2.1-17)
$ cat b.cpp
namespace hide { struct Bar {  int y; }; }
struct Foo {  hide::Bar* b; };

inline void x() { hide::Bar b; }

static Foo aFoo;

void * f() {
  return aFoo.b;
}

int main() {
  f();
  return 0;
}
$ g++ -fdebug-types-section b.cpp -g
$ ./build/gdb/gdb -ex "b f" -ex run -ex "ptype hide::Bar" ./a.out
...
GNU gdb (GDB) 8.2.50.20180720-git
...
Reading symbols from ./a.out...done.
Breakpoint 1 at 0x1129: file b.cpp, line 11.
Starting program: /usr/local/google/home/saugustine/gdb/a.out

Breakpoint 1, Python Exception <type 'exceptions.NameError'> Installation
error: gdb.execute_unwinders function is missing:
f () at b.cpp:11
11  return aFoo.b;
No symbol "Bar" in namespace "hide".
(gdb)