[5.4] [Bootstrap] Do not pass -Ddispatch_DIR to the Yams CMake build (#3363)

[shahmishal]

With the version-bump of Yams in swiftlang/swift#36366, Yams 4.0.2 now actually expresses the dependency on Dispatch in its CMake config.

With the current behavior of passing -Ddispatch_DIR to its CMake build, we have the following problem on Linux:

• swift's build-script installs Dispatch into a just-built toolchain which we use to build SwiftPM, which will contain, among other things, the Dispatch .swiftmodule.
• The compiler workspace checkout of swift-corelibs-libdispatch also contains a copy of the Dispatch .swiftmodule.

Both of these will be found, leading to build failures like:

/home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/swift-nightly-install/usr/lib/swift/dispatch/module.modulemap:1:8: error: redefinition of module 'Dispatch'
19:37:47 module Dispatch {
19:37:47        ^
19:37:47 /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/swift-corelibs-libdispatch/dispatch/module.modulemap:1:8: note: previously defined here
19:37:47 module Dispatch {
19:37:47        ^
19:37:47

We also cannot put off building libDispatch until SwiftPM is built, because the libDispatch dylib is required to link SwiftPM.
Not passing -Ddispatch_DIR to Yams' CMake build causes it to successfully locate the Dispatch package in the just-built toolchain on its own.

(cherry picked from commit 6af862d)

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[shahmishal]

@swift-ci test

[abertelrud]

Seems fine to me, though I don't have a lot of context for why this was there in the first place (other than your description, which is very helpful).

[shahmishal]

@swift-ci test

[shahmishal]

@swift-ci test