[embedded] Link in @_used declarations from other modules in SILLinker

[kubamracek]

This is a fix for #77812. In the Embedded Swift linkage model, the client/application module only links in what is actively used from other modules. There are situations where this is undesirable, and so this PR now explicitly links in all @_used functions and global variables from imported modules.

rdar://141239267

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test Windows platform

[kubamracek]

@swift-ci please test

[kubamracek]

Windows build fails with

FAILED: tools/swift/lib/SILOptimizer/CMakeFiles/swiftSILOptimizer.dir/UtilityPasses/Link.cpp.obj 
...
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/Runtime/RuntimeFunctions.def(2133): error C2872: 'Concurrency': ambiguous symbol
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\\include\ppltasks.h(95): note: could be 'Concurrency'
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/SIL/RuntimeEffect.h(92): note: or       'const swift::RuntimeEffect RuntimeConstants::Concurrency'
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/Runtime/RuntimeFunctions.def(2133): error C2882: 'Concurrency': illegal use of namespace identifier in expression
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/Runtime/RuntimeFunctions.def(2308): error C2872: 'Concurrency': ambiguous symbol
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.42.34433\\include\ppltasks.h(95): note: could be 'Concurrency'
C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\include\swift/SIL/RuntimeEffect.h(92): note: or       'const swift::RuntimeEffect RuntimeConstants::Concurrency'

[eeckstein]

lgtm

[MaxDesiatov]

@swift-ci test windows

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[MaxDesiatov]

@swift-ci test macos