[embedded] Fix embedded Swift builds with -Xcc -ffreestanding and -fno-builtin

[kubamracek]

The attached testcase shows the problem: With -Xcc -fno-builtin, or with -Xcc -ffreestanding, the compiler crashes during deserialization due to the "free" function not showing up in SwiftShims. This turns out to be caused by the declaration of free not being at the top level in LibcShims.h.

lib/swift/embedded/Swift.swiftmodule/arm64-apple-macos.swiftmodule:1:1: error: reference to top-level declaration 'free' broken by a context change; 'free' is not found, it was expected to be in 'SwiftShims'
SwiftShims.free
^
<unknown>:0: note: the declaration was expected to be found in module 'SwiftShims'
<unknown>:0: note: declarations in the  clang module 'SwiftShims' may be hidden by clang preprocessor macros
Stack dump:
3.	While evaluating request ExecuteSILPipelineRequest(Run pipelines { Mandatory Diagnostic Passes + Enabling Optimization Passes } on SIL for main)
4.	While running pass #51 SILModuleTransform "MandatorySILLinker".
5.	While deserializing SIL function "swift_deallocClassInstance"
6.	While deserializing SIL function "free"
7.	*** DESERIALIZATION FAILURE ***

[kubamracek]

@swift-ci please test

[mikeash]

🤨

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test Linux platform

[kubamracek]

@swift-ci please test Linux platform

[kubamracek]

@swift-ci please test Linux platform