swift234

[driver] Allow -force-single-frontend-invocation and -emit-objc-header[-path] to be used together.

Added support in Driver which allows -force-single-frontend-invocation and
-emit-objc-header[-path] to be used together in a single invocation.

Added support in tools::Swift to pass -emit-objc-header-path if an Objective-C
header was requested; this is only valid in OutputInfo::Mode::SingleCompile
mode, and an assertion enforces this.

Added a test which ensures that the same header is emitted in with
-force-single-frontend-invocation and without it.

Swift SVN r20185