[cxx-interop] Make usages of Swift Span @_alwaysEmitIntoClient in the overlay

This fixes a regression where projects that use the C++ stdlib overlay stop building because of a linker error:
```
ld: Shared cache eligible dylib cannot link to ineligible dylib '@rpath/libswiftCompatibilitySpan.dylib'.
```

Usages of `Span<T>` would generally cause a type metadata accessor to be emitted for `Swift.Span`. This becomes a problem with backdeployment, since Span is partially defined in a compatibility binary.

This change adds `@_alwaysEmitIntoClient` to generic usages of `Span` to prevent the type metadata accessor from being emitted into `libswiftCxx.a`.

rdar://152192080

Author: egorzhdan

stdlib/public/Cxx/CxxSpan.swift

@@ -74,22 +74,22 @@ public protocol CxxSpan<Element> {
 
 extension CxxSpan {
   /// Creates a C++ span from a Swift UnsafeBufferPointer
-  @inlinable
+  @_alwaysEmitIntoClient
   public init(_ unsafeBufferPointer: UnsafeBufferPointer<Element>) {
     unsafe precondition(unsafeBufferPointer.baseAddress != nil, 
                   "UnsafeBufferPointer should not point to nil")
     unsafe self.init(unsafeBufferPointer.baseAddress!, Size(unsafeBufferPointer.count))
   }
 
-  @inlinable
+  @_alwaysEmitIntoClient
   public init(_ unsafeMutableBufferPointer: UnsafeMutableBufferPointer<Element>) {
     unsafe precondition(unsafeMutableBufferPointer.baseAddress != nil, 
                   "UnsafeMutableBufferPointer should not point to nil")
     unsafe self.init(unsafeMutableBufferPointer.baseAddress!, Size(unsafeMutableBufferPointer.count))
   }
 
   @available(SwiftCompatibilitySpan 5.0, *)
-  @inlinable
+  @_alwaysEmitIntoClient
   @unsafe
   public init(_ span: Span<Element>) {
     let (p, c) = unsafe unsafeBitCast(span, to: (UnsafeRawPointer?, Int).self)
@@ -144,15 +144,15 @@ public protocol CxxMutableSpan<Element> {
 
 extension CxxMutableSpan {
   /// Creates a C++ span from a Swift UnsafeMutableBufferPointer
-  @inlinable
+  @_alwaysEmitIntoClient
   public init(_ unsafeMutableBufferPointer: UnsafeMutableBufferPointer<Element>) {
     unsafe precondition(unsafeMutableBufferPointer.baseAddress != nil, 
                   "UnsafeMutableBufferPointer should not point to nil")
     unsafe self.init(unsafeMutableBufferPointer.baseAddress!, Size(unsafeMutableBufferPointer.count))
   }
 
   @available(SwiftCompatibilitySpan 5.0, *)
-  @inlinable
+  @_alwaysEmitIntoClient
   @unsafe
   public init(_ span: consuming MutableSpan<Element>) {
     let (p, c) = unsafe unsafeBitCast(span, to: (UnsafeMutableRawPointer?, Int).self)