[Driver] Force the use of merge-module in merge-module specific test

As support, accept but ignore the -no-emit-module-separately flag in the
C++ driver.

Author: xymus

include/swift/Option/Options.td

@@ -593,6 +593,11 @@ def experimental_emit_module_separately:
   Flags<[NoInteractiveOption, HelpHidden]>,
   HelpText<"Emit module files as a distinct job (new Driver only)">;
 
+def no_emit_module_separately:
+  Flag<["-"], "no-emit-module-separately">,
+  Flags<[NoInteractiveOption, HelpHidden]>,
+  HelpText<"Force using merge-module as the incremental build mode (new Driver only)">;
+
 def requirement_machine_EQ : Joined<["-"], "requirement-machine=">,
   Flags<[FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Control usage of experimental generics implementation: 'on', 'off', or 'verify'">;

test/Driver/Dependencies/one-way-merge-module-fine.swift

@@ -4,7 +4,7 @@
 // RUN: cp -r %S/Inputs/one-way-fine/* %t
 // RUN: touch -t 201401240005 %t/*
 
-// RUN: cd %t && %swiftc_driver -driver-use-frontend-path "%{python.unquoted};%S/Inputs/update-dependencies.py;%swift-dependency-tool" -output-file-map %t/output.json -incremental -driver-always-rebuild-dependents ./main.swift ./other.swift -emit-module-path %t/master.swiftmodule -module-name main -j1 -v 2>&1 | %FileCheck -check-prefix=CHECK-FIRST %s
+// RUN: cd %t && %swiftc_driver -driver-use-frontend-path "%{python.unquoted};%S/Inputs/update-dependencies.py;%swift-dependency-tool" -output-file-map %t/output.json -incremental -driver-always-rebuild-dependents ./main.swift ./other.swift -emit-module-path %t/master.swiftmodule -module-name main -j1 -v -no-emit-module-separately 2>&1 | %FileCheck -check-prefix=CHECK-FIRST %s
 
 // CHECK-FIRST-NOT: warning
 // CHECK-FIRST: Handled main.swift
@@ -14,7 +14,7 @@
 // swift-driver checks existence of all outputs
 // RUN: touch -t 201401240006 %t/{main,other,master}.swift{module,doc,sourceinfo}
 
-// RUN: cd %t && %swiftc_driver -driver-use-frontend-path "%{python.unquoted};%S/Inputs/update-dependencies.py;%swift-dependency-tool" -output-file-map %t/output.json -incremental -driver-always-rebuild-dependents ./main.swift ./other.swift -emit-module-path %t/master.swiftmodule -module-name main -j1 -v 2>&1 | %FileCheck -check-prefix=CHECK-SECOND %s
+// RUN: cd %t && %swiftc_driver -driver-use-frontend-path "%{python.unquoted};%S/Inputs/update-dependencies.py;%swift-dependency-tool" -output-file-map %t/output.json -incremental -driver-always-rebuild-dependents ./main.swift ./other.swift -emit-module-path %t/master.swiftmodule -module-name main -j1 -v -no-emit-module-separately 2>&1 | %FileCheck -check-prefix=CHECK-SECOND %s
 
 // CHECK-SECOND-NOT: warning
 // CHECK-SECOND-NOT: Handled