Fixes to compile commands file watchers fallback logic

[yiftahw]

• closes Compile commands file watchers are leaked without being closed #12946
  • File watchers are closed before being disposed.
• closes Compile commands fallback logic can miss changes when using multiple configurations with different compile_commands.json paths #12947
  • Changed the fallback timestamp CppProperties.compileCommandsFileWatcherFallbackTime to be a Map<string, Date> which keeps track of a timestamp per file instead of a single one.