[Driver] Fix naming conflicts of getStatsFileName when using LTO

[dtcxzyw]

Summary

When using clang with -flto=thin -save-stats=obj to compile a multi-file program, clang will save internal statistics during the link time code generation to <obj_dir>/.stats because BaseName is empty. When multiple binaries are placed in the same directory, conflicts will be caused by identical filenames for statistics. This patch uses the output filename as the base name instead of the input when -save-stats=obj.

[dtcxzyw]

Fix test errors.