[Macros] Load '-load-plugin-library' plugins lazily
* Argument to '-load-plugin-library' now must have a filename that's
'{libprefix}{modulename}.{sharedlibraryextension}'
* Load '-load-plugin-library' plugins are now lazily loaded in
'CompilerPluginLoadRequest'
* Remove ASTContext.LoadedSymbols cache because they are cached by
'ExternalMacroDefinitionRequest' anyway
* `-load-plugin-executable` format validation is now in
'ParseSearchPathArgs'
