Disable IRGen tests to unblock CI:
Seeing failures in:
```
19:37:18 Failed Tests (3):
19:37:18 Swift(macosx-x86_64) :: IRGen/virtual-function-elimination-exec.swift
19:37:18 Swift(macosx-x86_64) :: IRGen/witness-method-elimination-exec.swift
19:37:18 Swift(macosx-x86_64) :: IRGen/witness-method-elimination-two-modules.swift
```
e.g. https://ci.swift.org/job/oss-swift_tools-RA_stdlib-RD_test-simulator//4847/console
Tracking in rdar://84643923
