[Driver] Add --config= as canonical spelling of --config

[MaskRay]

Summary

Driver options usually use Joined instead of Separate. It is also weird that
--config-system-dir=/etc exist while --config= did not exist.

[MaskRay]

.

[MaskRay]

--config is deliberately not a CoreOption as we want to discourage the form. --config was not CoreOption in previous releases.

[mgorny]

I don't know the fancy option magic you're using here, I haven't tested it but it seems to make sense to me.

clang/docs/UsersManual.rst

 specified just by referencing the configuration file. They may be used, for
 example, to collect options required to tune compilation for particular
 target, such as ``-L``, ``-I``, ``-l``, ``--sysroot``, codegen options, etc.
 
 Configuration files can be either specified on the command line or loaded
 from default locations. If both variants are present, the default configuration
 files are loaded first.
 
-The command line option ``--config`` can be used to specify explicit
+The command line option ``--config=`` can be used to specify explicit
 configuration files in a Clang invocation. If the option is used multiple times,
 all specified files are loaded, in order. For example:
 
 ::
 
-    clang --config /home/user/cfgs/testing.txt
-    clang --config debug.cfg --config runtimes.cfg
+    clang --config=/home/user/cfgs/testing.txt
+    clang --config=debug.cfg --config=runtimes.cfg
 
 If the provided argument contains a directory separator, it is considered as
 a file path, and options are read from that file. Otherwise the argument is
 treated as a file name and is searched for sequentially in the directories:
 
     - user directory,
     - system directory,
     - the directory where Clang executable resides.

 
 To generate paths relative to the configuration file, the ``<CFGDIR>`` token may
 be used. This will expand to the absolute path of the directory containing the
 configuration file.
 
 In cases where a configuration file is deployed alongside SDK contents, the
 SDK directory can remain fully portable by using ``<CFGDIR>`` prefixed paths.
 In this way, the user may only need to specify a root configuration file with
-``--config`` to establish every aspect of the SDK with the compiler:
+``--config=`` to establish every aspect of the SDK with the compiler:
 
 ::
 
     --target=foo
     -isystem <CFGDIR>/include
     -L <CFGDIR>/lib
     -T <CFGDIR>/ldscripts/link.ld
 

clang/include/clang/Driver/Options.td

 def cl_ext_EQ : CommaJoined<["-"], "cl-ext=">, Group<opencl_Group>, Flags<[CC1Option]>,
   HelpText<"OpenCL only. Enable or disable OpenCL extensions/optional features. The argument is a comma-separated "
            "sequence of one or more extension names, each prefixed by '+' or '-'.">,
   MarshallingInfoStringVector<TargetOpts<"OpenCLExtensionsAsWritten">>;
 
 def client__name : JoinedOrSeparate<["-"], "client_name">;
 def combine : Flag<["-", "--"], "combine">, Flags<[NoXarchOption, Unsupported]>;
 def compatibility__version : JoinedOrSeparate<["-"], "compatibility_version">;
-def config : Separate<["--"], "config">, Flags<[NoXarchOption, CoreOption]>,
-  HelpText<"Specifies configuration file">;
+def config : Joined<["--"], "config=">, Flags<[NoXarchOption, CoreOption]>, MetaVarName<"<file>">,
+  HelpText<"Specify configuration file">;
+def : Separate<["--"], "config">, Alias<config>;
 def no_default_config : Flag<["--"], "no-default-config">, Flags<[NoXarchOption, CoreOption]>,
   HelpText<"Disable loading default configuration files">;
 def config_system_dir_EQ : Joined<["--"], "config-system-dir=">, Flags<[NoXarchOption, CoreOption, HelpHidden]>,
   HelpText<"System directory for configuration files">;
 def config_user_dir_EQ : Joined<["--"], "config-user-dir=">, Flags<[NoXarchOption, CoreOption, HelpHidden]>,
   HelpText<"User directory for configuration files">;
 def coverage : Flag<["-", "--"], "coverage">, Group<Link_Group>, Flags<[CoreOption]>;
 def cpp_precomp : Flag<["-"], "cpp-precomp">, Group<clang_ignored_f_Group>;

clang/test/Driver/config-file.c

 //--- Config file search directories
 //
 // RUN: %clang --config-system-dir=%S/Inputs/config --config-user-dir=%S/Inputs/config2 -o /dev/null -v 2>&1 | FileCheck %s -check-prefix CHECK-DIRS
 // CHECK-DIRS: System configuration file directory: {{.*}}/Inputs/config
 // CHECK-DIRS: User configuration file directory: {{.*}}/Inputs/config2
 
 
 //--- Config file (full path) in output of -###
 //
 // RUN: %clang --config %S/Inputs/config-1.cfg -S %s -### 2>&1 | FileCheck %s -check-prefix CHECK-HHH
+// RUN: %clang --config=%S/Inputs/config-1.cfg -S %s -### 2>&1 | FileCheck %s -check-prefix CHECK-HHH
 // CHECK-HHH: Configuration file: {{.*}}Inputs{{.}}config-1.cfg
 // CHECK-HHH: -Werror
 // CHECK-HHH: -std=c99
 
 
 //--- Config file (full path) in output of -v
 //
 // RUN: %clang --config %S/Inputs/config-1.cfg -S %s -o /dev/null -v 2>&1 | FileCheck %s -check-prefix CHECK-V