[lldb] Tab completion for process load/unload

1. Complete `process load` with the common disk file completion, so there is not test provided for it;
2. Complete `process unload` with the tokens of valid loaded images.

Thanks for Raphael's help on the test for `process unload`.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D79887

lldb/test/API/functionalities/completion/shared.cpp

@@ -0,0 +1,3 @@
+int foo() {
+  return 123;
+}