[clang] Add '-ast-dump-filter=' support

Before this patch, we only support syntax like
`clang -cc1 -ast-dump -ast-dump-filter main a.c`
or
`clang -Xclang -ast-dump -Xclang -ast-dump-filter -Xclang main a.c`
when using ast-dump-filter.

It is helpful to also support `-ast-dump-filter=` syntax, so we can do
something like
`clang -cc1 -ast-dump -ast-dump-filter=main a.c`
or
`clang -Xclang -ast-dump -Xclang -ast-dump-filter=main a.c`

It is more cleaner when passing arguments through `-Xclang` in this case.

Also, **clang-check** do support this syntax, and I think people might
be confiused when they found they can't use `ast-dump-filter` with
clang.

clang/include/clang/Driver/Options.td

@@ -5282,6 +5282,8 @@ def ast_dump_filter : Separate<["-"], "ast-dump-filter">,
            " nodes having a certain substring in a qualified name. Use"
            " -ast-list to list all filterable declaration node names.">,
   MarshallingInfoString<FrontendOpts<"ASTDumpFilter">>;
+def ast_dump_filter_EQ : Joined<["-"], "ast-dump-filter=">,
+  Alias<ast_dump_filter>;
 def fno_modules_global_index : Flag<["-"], "fno-modules-global-index">,
   HelpText<"Do not automatically generate or update the global module index">,
   MarshallingInfoNegativeFlag<FrontendOpts<"UseGlobalModuleIndex">>;