I get the following error:

Compiling the Swift compiler identification source file "CompilerId/main.swift" failed.
Compiler:  
Build flags: 
Id flags: 

The output was:
65
=== BUILD TARGET CompilerIdSwift OF PROJECT CompilerIdSwift WITH THE DEFAULT CONFIGURATION (Debug) ===

Check dependencies
“Use Legacy Swift Language Version” (SWIFT_VERSION) is required to be configured correctly for targets which use Swift. Use the [Edit > Convert > To Current Swift Syntax…] menu to choose a Swift version or use the Build Settings editor to configure the build setting directly.

** BUILD FAILED **

The trick is to specify the required Swift version in the Xcode project:

                        isa = XCBuildConfiguration;
                        buildSettings = {
                                PRODUCT_NAME = CompilerIdSwift;
+                               SWIFT_VERSION = 2.3;
                        };