[gdb/symtab] Handle invalid partial DIE reference

[gdb/symtab] Handle invalid partial DIE reference

When reverting commit 9cfd2b89bd "[gdb/testsuite] Fix
gdb.arch/amd64-entry-value-paramref.S", we run into an internal-error:
...
(gdb) file amd64-entry-value-paramref^M
Reading symbols from amd64-entry-value-paramref...^M
src/gdb/dwarf2/read.c:18903: internal-error: could not find partial DIE
  0x1b7 in cache [from module amd64-entry-value-paramref]^M

A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
...
because of invalid dwarf.

In contrast, when using -readnow, we have:
...
(gdb) file -readnow amd64-entry-value-paramref
Reading symbols from amd64-entry-value-paramref...
Expanding full symbols from amd64-entry-value-paramref...
Dwarf Error: Cannot find DIE at 0x1b7 referenced from DIE at 0x11a \
  [in module amd64-entry-value-paramref]
(gdb)
...

Change the internal error into a Dwarf Error, such that we have:
...
(gdb) file amd64-entry-value-paramref^M
Reading symbols from amd64-entry-value-paramref...^M
Dwarf Error: Cannot not find DIE at 0x1b7 \
  [from module amd64-entry-value-paramref]^M
^M
(No debugging symbols found in amd64-entry-value-paramref)^M
(gdb)
...

Build and tested on x86_64-linux.

gdb/ChangeLog:

2020-08-04  Tom de Vries  <tdevries@suse.de>

	PR symtab/23270
	* dwarf2/read.c (find_partial_die): Change internal error into Dwarf
	Error.