Add Builtin.ifdef_<FLAGNAME> as a facility to peek at -D flag that client code is building with (#39797)

test/stdlib/builtin-ifdef.swift

@@ -0,0 +1,34 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -parse-stdlib -whole-module-optimization -D LIBRARY -emit-module -o %t/Library.swiftmodule
+// RUN: %target-build-swift %s -parse-stdlib -whole-module-optimization -D LIBRARY -emit-bc -o %t/Library.bc
+// RUN: %target-build-swift %s -parse-as-library -D CLIENT -I %t -o %t/R1 -O            && %target-run %t/R1 | %FileCheck %s
+// RUN: %target-build-swift %s -parse-as-library -D CLIENT -I %t -o %t/R2 -O -D FOO_BAR && %target-run %t/R2 | %FileCheck %s --check-prefix CHECK-FOO-BAR
+
+// REQUIRES: executable_test
+
+#if LIBRARY
+
+  @_alwaysEmitIntoClient
+  public func ifdefFooBar() -> Builtin.Int1 {
+    return Builtin.ifdef_FOO_BAR()
+  }
+
+#endif
+
+#if CLIENT
+
+  import Library
+
+  @_cdecl("main")
+  func main() -> Int32 {
+    print("Hello")
+    print(Bool(_builtinBooleanLiteral: ifdefFooBar()))
+    // CHECK: Hello
+    // CHECK: false
+
+    // CHECK-FOO-BAR: Hello
+    // CHECK-FOO-BAR: true
+    return 0
+  }
+
+#endif