The value of the "addr" field for a pending lldb-mi breakpoint is
"0xffffffffffffffff". MIBreakpointController::updateFromDebugger()
compares the "addr" field value to the differing GDB/MI's pending value
of "<PENDING>". Therefore, pending lldb-mi breakpoints are never
detected. This is why breakpoints are never displayed as pending in the
UI during a LLDB debug session. And test_lldb functions that check the
pending state are either unconditionally skipped or XFAIL.

Introduce a new virtual function
MIBreakpointController::pendingBreakpointAddress() and override it in
GDB::BreakpointController and LLDB::BreakpointController to detect all
pending breakpoints. Now pending breakpoints are displayed as such in
the UI during any debug session. LldbTest::testPendingBreakpoint() and
LldbTest::testBreakpointsOnNoOpLines() fully pass.

TestDebugSession is a dummy debug session, which does not actually use
any machine interface and therefore never invokes
pendingBreakpointAddress(). Derive a trivial class
TestMIBreakpointController from MIBreakpointController and return an
empty string from its pendingBreakpointAddress(); use the new class in
TestDebugSession instead of the now-abstract MIBreakpointController.