[WIP] Enable support for the `@_section` experimental attribute.

[grynspan]

This PR enables support for @_used and @_section to allow embedding test data directly into its own section in a binary. This means we don't have to look up synthesized test types at runtime.

Limitations:

1. Test clients must enable support for the experimental "SymbolLinkageMarkers" feature.
2. The @_section attribute currently doesn't support application to constants of function type, thinking they are non-constant.
3. We will need to make a change to the Swift runtime on Linux/Windows to add the new section to MetadataSections (or alternatively, the experimental feature needs to provide a different way to discover sections at runtime.) See https://github.com/apple/swift/pull/71509.

:warning: Do not merge: this is a proof-of-concept and work-in-progress.

Checklist:

• [x] Code and documentation should follow the style of the Style Guide.
• [x] If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci please test

[grynspan]

@swift-ci please test

[grynspan]

@compnerd says the Windows section needs the suffix "$B".

[grynspan]

@swift-ci please test

[grynspan]

Linux failure is expected right now; the new swift5_tests section needs a newer toolchain.

[grynspan]

Looks like Windows has a compile-time complaint about the constness of C function pointers.

[grynspan]

@swift-ci please test

[grynspan]

@swift-ci please test