The change in 269fc941 turned out not to be enough to solve linker errors when using the CxxStdlib overlay with a non-default C++ stdlib.
In addition to `@inlinable`, the Swift functions in the overlay need to be `@_alwaysEmitIntoClient` to prevent Swift from trying to link a program that uses libc++ on Linux against the CxxStdlib binary shipped in the toolchain.
rdar://138838506
This is a requirement for being able to use the CxxStdlib overlay with a C++ standard library which is not the platform default, specifically libc++ on Linux.
The module would get rebuilt from its textual interface (`CxxStdlib.swiftinterface`) by the compiler whenever a custom C++ stdlib is used. Since the mangled names of C++ stdlib symbols differ across implementations (libc++ vs libstdc++), a Swift program that uses these overlay functions would fail to link, unless the definitions of these functions are available.
rdar://138838506
