build: fallback BOOTSTRAPPING_MODE before the first use

[kateinoigakukun]

BOOTSTRAPPING_MODE was used before it's fully determined by a falling back logic (the fallback is introduced recently by 9017ef5).

It results in validation test failure with --bootstrapping=bootstrapping --enable-array-cow-checks on Linux, which is used by buildbot_incremental_linux_crosscompile_wasm¹.

This PR just moves the code block for loading earlyswiftsyntax targets and falling back BOOTSTRAPPING_MODE before the first use of the variable.

Footnotes

1. https://ci.swift.org/job/swift-PR-Linux-preset/69 ↩

[kateinoigakukun]

preset=buildbot_incremental_linux_crosscompile_wasm
@swift-ci Please test with preset Linux Platform

[kateinoigakukun]

preset=buildbot_incremental,tools=RA,stdlib=RA
@swift-ci Please clean test with preset macOS

[kateinoigakukun]

@swift-ci please smoke test

[kateinoigakukun]

Still the preset job is failing due to another issue, but #68538 will fix it.

[MaxDesiatov]

preset=buildbot_incremental_linux_crosscompile_wasm
@swift-ci Please test with preset Linux Platform

[rintaro]

OK, but

It results in validation test failure with --bootstrapping=bootstrapping --enable-array-cow-checks on Linux, which is used by buildbot_incremental_linux_crosscompile_wasm

Now that --bootstrapping=bootstrapping is not supported in linux, so SWIFT_ENABLE_ARRAY_COW_CHECKS is now practically always FALSE? Is it OK?

[kateinoigakukun]

Yes, this is intended to be a quick fix to unblock other PRs. Will update the presets themselves separately 👍