Only configure swift-reflection-dump if the host SDK is requested

[bitjammer]

Since swift-reflection-dump is a host-only tool, for iOS-only builds or
other builds where the host SDK isn't configured, the
swift-reflection-dump tool will fail to find libswiftReflection.

rdar://problem/25858390

[bitjammer]

@gribozavr Does this change look sane to you?

[bitjammer]

cc @egranata

[modocache]

Looks good to me, although I defer to @gribozavr's judgement.

One tiny nit-pick: other uses of is_sdk_requested in the codebase store the result in a lowercase variable like swift_build_tvos. I think the (loosely followed) convention is ALL_CAPS for variables that are meant to be global, lowercase otherwise.

[bitjammer]

Yeah, I meant to make this a global since we'll probably use it elsewhere for these mixed tools, but it should probably move up in the file with a comment, now that you mention it.

[bitjammer]

Actually, I think the check needs to be even more restrictive, I think it might need to be SWIFT_BUILD_TOOLS AND SWIFT_BUILD_STDLIB AND SWIFT_HOST_SDK_REQUESTED.

[gribozavr]

LGTM with the more restrictive condition.

[bitjammer]

@swift-ci Please test

[bitjammer]

Unrelated failure - merging.