[Sema] @_spi decls don't require an availability attribute

Author: xymus

lib/Sema/TypeCheckAvailability.cpp

@@ -2866,7 +2866,9 @@ static bool declNeedsExplicitAvailability(const Decl *decl) {
       return false;
   }
 
-  if (decl->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>())
+  // Skip functions emitted into clients or SPI.
+  if (decl->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>() ||
+      decl->isSPI())
     return false;
 
   // Warn on decls without an introduction version.

test/attr/require_explicit_availability.swift

@@ -79,3 +79,15 @@ public protocol PublicProtocol { }
 
 @available(macOS 10.1, *)
 extension S : PublicProtocol { }
+
+@_spi(SPIsAreOK)
+public func spiFunc() { }
+
+@_spi(SPIsAreOK)
+public struct spiStruct {
+  public func spiMethod() {}
+}
+
+extension spiStruct {
+  public func spiExtensionMethod() {}
+}