Add new gdb.ada/bp_c_mixed_case testcase for PR gdb/22670

Add new gdb.ada/bp_c_mixed_case testcase for PR gdb/22670

This patch adds a new testcase to demonstrate a regression introduced by:

    commit b5ec771e60c1a0863e51eb491c85c674097e9e13
    Date:   Wed Nov 8 14:22:32 2017 +0000
    Subject: Introduce lookup_name_info and generalize Ada's FULL/WILD name matching

The purpose of the testcase is to verify that a user can insert
a breakpoint on a C function while debugging Ada, even if the name
of the function includes uppercase letters, requiring us to use
Ada's "<...>" notation to tell the GDB that the symbol name should
be looked up verbatim.

As of the commit above, GDB is no longer finding the function:

    (gdb) break <MixedCaseFunc>
    Function "<MixedCaseFunc>" not defined.
    Make breakpoint pending on future shared library load? (y or [n])

Before the patch, the breakpoint was inserted without problem.

gdb/testsuite/ChangeLog:

        PR gdb/22670
        * gdb.ada/bp_c_mixed_case: New testcase.

Tested on x86_64-linux; generates a KPASS before the regression
was introduced, and now generates a KFAIL.