Add support for displaying the language in the frame-format string.

Enable ${language} to be specified in the frame-format string to see
the current frame's compile unit language in "frame info".

Test Plan:
debug a C++ program, run to main, and run the lldb commands:
    settings set frame-format "frame lang=${language}\n"
    frame info
you should see:
    frame lang=c++
test case added in:
    ./dotest.py --executable lldb -f SettingsCommandTestCase.test_set_frame_format
Reviewers: clayborg
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D10561

llvm-svn: 240440