[gdb/testsuite] Add gdb.dwarf2/imported-unit-c.exp

[gdb/testsuite] Add gdb.dwarf2/imported-unit-c.exp

This test-case is intended to excercise 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;
...
in the sense that the test-case should fail if the
"per_cu->lang == language_cplus" clause is removed.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-22  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/imported-unit-c.exp: New file.