XCTest Overlay: Add XCTAssertThrowsError assertion
XCTest Overlay: Add XCTAssertThrowsError assertion

Add an assertion that can be used to check that an expression throws an
error, and if it does to run a block against that error which may
contain further checks.

Addresses rdar://problem/23789904.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift