Runtimes: introduce SwiftOnoneSupport

[edymtt]

The main goal for now is to have this building, with little concern about matching the build flags used currently.

Keep it disabled by default on Apple platforms.

Addresses rdar://143151393

[edymtt]

@swift-ci please smoke test

[edymtt]

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[edymtt]

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[etcwilde]

Looking for the missing flag, but it looks like there are ~1300 missing public symbols.

[etcwilde]

It looks like we're at least missing -Xfrontend -prespecialize-generic-metadata. That doesn't seem to get all of the generic specializations back though so we're likely missing more.

[edymtt]

I could comb through the CMake logic to find the missing flags, but it may be more efficient to ensure the stdlib build invoked with ExternalProject_Add has verbose output in CI and comb the resulting log -- opened #79174 as a way to achieve that

[edymtt]

I've landed #79188 and #79266 to account for the missing flags.

[edymtt]

@swift-ci please smoke test

[edymtt]

@swift-ci please smoke test Linux

[edymtt]

@swift-ci please smoke test Windows

[edymtt]

From a macOS build at desk, it looks like the SwiftOnoneSupport built this way generates the same symbols as the one build the old way (except the magic install name symbols, since we are not setting SwiftCore_ENABLE_BACKDEPLOYMENT_SUPPORT when configuring this build)

> diff -u <(nm -arch arm64 -g lib/swift/macosx/libswiftSwiftOnoneSupport.dylib| awk '{print $3}' | sort -u | xcrun swift-demangle) <(nm -arch arm64 -g swift-stdlib-macosx-arm64-core-prefix/src/swift-stdlib-macosx-arm64-core-build/SwiftOnoneSupport/libswiftSwiftOnoneSupport.dylib| awk '{print $3}'  | sort -u | xcrun swift-demangle) 
--- /dev/fd/11	2025-02-12 08:20:11
+++ /dev/fd/12	2025-02-12 08:20:11
@@ -1,5 +1,4 @@
 
-$ld$previous$@rpath/libswiftSwiftOnoneSupport.dylib$$1$10.9$10.14.4$$
 generic specialization <Swift.Character> of Swift.Array.init() -> [A]
 generic specialization <Swift.String.UnicodeScalarView> of Swift.Array.init() -> [A]
 generic specialization <Swift.String.UTF8View> of Swift.Array.init() -> [A]

[compnerd]

Noticed some bleeding whitespace

[edymtt]

@swift-ci please smoke test