jit.exp: support C++ testcases

jit.exp: support C++ testcases

gcc/jit/ChangeLog:
	* TODO.rst (Test suite): Remove item about running C++ testcases.
	* docs/internals/index.rst (Working on the JIT library): Add
	"c++" to the enabled languages in the suggested "configure"
	invocation, and add a description of why this is necessary.
	* docs/_build/texinfo/libgccjit.texi: Regenerate.

gcc/testsuite/ChangeLog:
	* jit.dg/jit.exp: Load wrapper.exp with %{tool} set to "g++"
	rather than "jit".  Load g++.exp, and call g++_init.
	Run test-*.cc files within the testsuite and *.cc files within
	docs/examples.
	(jit-dg-test): Drop the addition of -fgnu89-inline to
	DEFAULT_CFLAGS in favor of adding it to additional_flags, only
	doing it when compiling C testcases (since g++ does not handle
	it).  Reset "orig_environment_saved" so that LD_LIBRARY_PATH
	is restored to the value after g++_init ran, rather than the
	value before g++_init ran.  Return a list of
	$comp_output $output_file, as dg-test assumes.

From-SVN: r219318