gdb/testsuite/fortran: Add mixed language stack test

This commit adds a test that builds a mixed language stack, the stack
contains frames of Fortran, C, and C++.  The test prints the backtrace
and explores the stack printing arguments of different types in frames
of different languages.

The core of the test is repeated with GDB's language set to auto,
fortran, c, and c++ in turn to ensure that GDB is happy to print
frames and frame arguments when the language is set to a value that
doesn't match the frame language.

This test currently passes, and there are no known bugs in this area.
The aim of this commit is simply to increase test coverage, as I don't
believe this functionality is currently tested.

gdb/testsuite/ChangeLog:

	* gdb.fortran/mixed-lang-stack.c: New file.
	* gdb.fortran/mixed-lang-stack.cpp: New file.
	* gdb.fortran/mixed-lang-stack.exp: New file.
	* gdb.fortran/mixed-lang-stack.f90: New file.