[gdb/symtab] Fix skipping of import of C++ CU

Tom Tromey observed that when changing the language in
gdb.dwarf2/imported-unit-bp.exp from c to c++, the test failed.

This is due to this code in process_imported_unit_die:
...
      /* We're importing a C++ compilation unit with tag DW_TAG_compile_unit
         into another compilation unit, at root level.  Regard this as a hint,
         and ignore it.  */
      if (die->parent && die->parent->parent == NULL
          && per_cu->unit_type == DW_UT_compile
          && per_cu->lang == language_cplus)
        return;
...
which should have a partial symtabs counterpart.

Add the missing counterpart in process_psymtab_comp_unit.

Tested on x86_64-linux (openSUSE Leap 15.2), no regressions for config:
- using default gcc version 7.5.0
  (with 5 unexpected FAILs)
- gcc 10.3.0 and target board
  unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects
  (with 1000 unexpected FAILs)

gdb/ChangeLog:

2021-07-06  Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c (scan_partial_symbols): Skip top-level imports of
	c++ CU.
	* testsuite/gdb.dwarf2/imported-unit-bp.exp: Moved to ...
	* testsuite/gdb.dwarf2/imported-unit-bp.exp.tcl: ... here.
	* testsuite/gdb.dwarf2/imported-unit-bp-c++.exp: New test.
	* testsuite/gdb.dwarf2/imported-unit-bp-c.exp: New test.
	* testsuite/gdb.dwarf2/imported-unit.exp: Update.

1 files changed, 7 insertions, 0 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index fe82175..a13a53e 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -7639,6 +7639,13 @@ scan_partial_symbols (struct partial_die_info *first_die, CORE_ADDR *lowpc,
 		  process_psymtab_comp_unit (per_cu, cu->per_objfile, true,
 					     cu->per_cu->lang);
 
+		if (pdi->die_parent == nullptr
+		    && per_cu->unit_type == DW_UT_compile
+		    && per_cu->lang == language_cplus)
+		  /* Regard import as hint.  See corresponding code in
+		     process_imported_unit_die.  */
+		  break;
+
 		cu->per_cu->imported_symtabs_push (per_cu);
 	      }
 	      break;