[SR-2507] Can't test optimized builds

[drewcrawford]

Previous ID	SR-2507
Radar	None
Original Reporter	@drewcrawford
Type	Bug

Environment

Linux x64
swift-3.0-PREVIEW-6

Additional Detail from JIRA

Votes	0
Component/s	XCTest
Labels	Bug, Linux, OptimizedOnly
Assignee	None
Priority	Medium

md5: e75c7ef8d15f885bc7339674089eb704

Issue Description:

$ cat test.swift
import XCTest

class Foo: XCTestCase {
}

$ swift test.swift
$ swift -O test.swift
LLVM ERROR: Program used external function '_TFC6XCTest10XCTestCasem17_performanceMeterGSqCS_16PerformanceMeter_' which could not be resolved!