gdb/23712: Test case for multidictionary

This is a test derived from one of the reproducers in symtab/23010.
The DIE tree used here is typical of compilations with LTO, where an
artificial parent DIE of language C99 imports DIEs of other languages.

gdb/testsuite/ChangeLog:

	PR gdb/23712
	PR symtab/23010
	* gdb.dwarf2/multidictionary.exp: New file.