[gdb/symtab] Don't create duplicate psymtab for forward-imported CU

Consider the executable generated for test-case gdb.dwarf2/imported-unit.exp.

When loading the executable using various tracing:
...
$ gdb \
  outputs/gdb.dwarf2/imported-unit/imported-unit \
  -batch \
  -iex "set verbose on" \
  -iex "set debug symtab-create 1"
  ...
Created psymtab 0x213f380 for module <artificial>@0xc7.
Created psymtab 0x20e7b00 for module imported_unit.c.
Created psymtab 0x215da20 for module imported_unit.c.
Created psymtab 0x2133630 for module elf-init.c.
Created psymtab 0x215b910 for module ../sysdeps/x86_64/crtn.S.
...
we notice that there are two psymtabs generated for imported_unit.c.

This is due to the following: in dwarf2_build_psymtabs_hard we loop over CUs
and generate partial symtabs for those, and if we encounter an import of
another CU, we also generate a partial symtab for that one, unless already
created.

This works well with backward import references:
- the imported CU is read
- then the importing CU is read
- the import is encountered, but the imported CU is already read, so
  we're done.

But with forward import references, we have instead:
- the importing CU is read
- the import is encountered, and the imported CU is read
- the imported CU is read once more

Fix this by skipping already created psymtabs in the loop in
dwarf2_build_psymtabs_hard.

Tested on x86_64-linux, with native and target board
unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects.

This causes this regression with the target board:
...
FAIL: gdb.ada/dgopt.exp: list x.adb:16, 16
...
which I consider a seperate PR, filed as PR25801 - "Filename of shared psymtab
is ignored".

gdb/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	PR symtab/25700
	* dwarf2/read.c (dwarf2_build_psymtabs_hard): Don't create psymtab for
	CU if already created.

gdb/testsuite/ChangeLog:

2020-04-22  Tom de Vries  <tdevries@suse.de>

	PR symtab/25700
	* gdb.dwarf2/imported-unit.exp: Verify that there's only one partial
	symtab for imported_unit.c.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-22  Tom de Vries  <tdevries@suse.de>
+
+	PR symtab/25700
+	* dwarf2/read.c (dwarf2_build_psymtabs_hard): Don't create psymtab for
+	CU if already created.
+
 2020-04-21  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
 
 	* infrun.c (displaced_step_fixup): Switch to the event_thread

gdb/dwarf2/read.c

@@ -7779,7 +7779,12 @@ dwarf2_build_psymtabs_hard (struct dwarf2_per_objfile *dwarf2_per_objfile)
 			   addrmap_create_mutable (&temp_obstack));
 
   for (dwarf2_per_cu_data *per_cu : dwarf2_per_objfile->all_comp_units)
-    process_psymtab_comp_unit (per_cu, false, language_minimal);
+    {
+      if (per_cu->v.psymtab != NULL)
+	/* In case a forward DW_TAG_imported_unit has read the CU already.  */
+	continue;
+      process_psymtab_comp_unit (per_cu, false, language_minimal);
+    }
 
   /* This has to wait until we read the CUs, we need the list of DWOs.  */
   process_skeletonless_type_units (dwarf2_per_objfile);

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-22  Tom de Vries  <tdevries@suse.de>
+
+	PR symtab/25700
+	* gdb.dwarf2/imported-unit.exp: Verify that there's only one partial
+	symtab for imported_unit.c.
+
 2020-04-21  Gary Benson <gbenson@redhat.com>
 
 	* gdb.base/advance.c (func): New argument, to match call site.

gdb/testsuite/gdb.dwarf2/imported-unit.exp

@@ -168,6 +168,24 @@ if { $psymtabs_p } {
 } else {
     unsupported $test
 }
+
+# Verify that there's only one partial symtab for imported_unit.c.  Test-case
+# for PR25700.
+set test "no duplicate psymtab for imported_unit.c"
+if { $psymtabs_p } {
+    set line "Partial symtab for source file imported_unit.c"
+    gdb_test_multiple "maint print psymbols" $test {
+	-re -wrap "$line.*$line.*" {
+	    fail $gdb_test_name
+	}
+	-re -wrap "$line.*" {
+	    pass $gdb_test_name
+	}
+    }
+} else {
+    unsupported $test
+}
+
 # Sanity check
 gdb_test "ptype main" "= int \\(void\\)"