Build lldb-mi

Building this tool requires having a full LLVM build available;
either by installing without LLVM_INSTALL_TOOLCHAIN_ONLY (and not
stripping the llvm installation yet) or by pointing it at the
LLVM build directory instead. The latter works better for our
normal build flows.

.dockerignore

@@ -2,6 +2,7 @@
 llvm-project
 mingw-w64
 libssp
+lldb-mi
 make-*
 **/*.exe
 **/*.dll