stdlib: add a _unsafePerformance function which disable performance diagnostics in the passed closure.

Author: eeckstein

include/swift/AST/SemanticAttrs.def

@@ -115,5 +115,7 @@ SEMANTICS_ATTR(OBJC_FORBID_ASSOCIATED_OBJECTS, "objc.forbidAssociatedObjects")
 
 SEMANTICS_ATTR(LIFETIMEMANAGEMENT_MOVE, "lifetimemanagement.move")
 
+SEMANTICS_ATTR(NO_PERFORMANCE_ANALYSIS, "no_performance_analysis")
+
 #undef SEMANTICS_ATTR
 

stdlib/public/core/Misc.swift

@@ -118,3 +118,10 @@ public func _getTypeByMangledNameInContext(
   genericContext: UnsafeRawPointer?,
   genericArguments: UnsafeRawPointer?)
   -> Any.Type?
+
+/// Prevents performance diagnostics in the passed closure.
+@_alwaysEmitIntoClient
+@_semantics("no_performance_analysis")
+public func _unsafePerformance<T>(_ c: () -> T) -> T {
+  return c()
+}