Unable to SIGINT swift-driver when reading from stdin

[rjmansfield]

The handling of the SIGINT in the driver doesn't allow the swift-driver to be interrupted with ctrl + c .

e.g. $ swiftc - Then ctrl + c in the same terminal, orkill -SIGINTpgrep swift-driver`` in another terminal, the driver won't exit and will have to be killed with another signal. e.g. SIGKILL

The main thread is blocked n llbuild

* thread #1, queue = 'com.apple.main-thread', stop reason = instruction step over
  * frame #0: 0x000000019a7703f4 libsystem_kernel.dylib`__psynch_cvwait + 36
    frame #1: 0x000000019a7ac520 libsystem_pthread.dylib`_pthread_cond_wait + 984
    frame #2: 0x000000019a6df298 libc++.1.dylib`std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 32
    frame #3: 0x00000001019cdd84 libllbuildSwift.dylib`(anonymous namespace)::BuildEngineImpl::executeTasks(this=0x0000000100febed0, buildKey=0x000000016fdf2fb0) at BuildEngine.cpp:1051:38
    frame #4: 0x00000001019bf8a8 libllbuildSwift.dylib`(anonymous namespace)::BuildEngineImpl::build(this=0x0000000100febed0, key=0x000000016fdf2fb0) at BuildEngine.cpp:1598:20
    frame #5: 0x00000001019bf460 libllbuildSwift.dylib`llbuild::core::BuildEngine::build(this=0x0000000b84260030, key=0x000000016fdf2fb0) at BuildEngine.cpp:1934:47
    frame #6: 0x00000001018eda34 libllbuildSwift.dylib`llb_buildengine_build(engine_p=0x0000000b84260020, key=0x000000016fdf3200, result_out=0x000000016fdf3620) at Core-C-API.cpp:216:26
    frame #7: 0x00000001018b6fe4 libllbuildSwift.dylib`closure #1 in closure #1 in BuildEngine.build($0=0x16fdf3200, resultPtr=0x16fdf3620) at CoreBindings.swift:407:39
    frame #11: 0x00000001018b43d8 libllbuildSwift.dylib`closure #1 in Key.withInternalDataPtr<()>(dataPtr=46 values (0xb84039b00), closure=0x00000001018b92c4 libllbuildSwift.dylib`partial apply forwarder for closure #1 (Swift.UnsafePointer<__C.llb_data_t_>) -> () in closure #1 (Swift.UnsafeMutablePointer<__C.llb_data_t_>) -> () in llbuildSwift.BuildEngine.build(key: llbuildSwift.Key) -> llbuildSwift.Value at <compiler-generated>) at CoreBindings.swift:63:20
    frame #15: 0x00000001018b4228 libllbuildSwift.dylib`Key.withInternalDataPtr<()>(closure=0x00000001018b92c4 libllbuildSwift.dylib`partial apply forwarder for closure #1 (Swift.UnsafePointer<__C.llb_data_t_>) -> () in closure #1 (Swift.UnsafeMutablePointer<__C.llb_data_t_>) -> () in llbuildSwift.BuildEngine.build(key: llbuildSwift.Key) -> llbuildSwift.Value at <compiler-generated>) at CoreBindings.swift:61:21
    frame #16: 0x00000001018b6f5c libllbuildSwift.dylib`closure #1 in BuildEngine.build(resultPtr=0x16fdf3620, key=llbuildSwift.Key @ 0x000000016fdf3500) at CoreBindings.swift:407:17
    frame #20: 0x00000001018b4f30 libllbuildSwift.dylib`static Value.fromInternalDataOutputPtr(closure=0x00000001018b8584 libllbuildSwift.dylib`partial apply forwarder for closure #1 (Swift.UnsafeMutablePointer<__C.llb_data_t_>) -> () in llbuildSwift.BuildEngine.build(key: llbuildSwift.Key) -> llbuildSwift.Value at <compiler-generated>) at CoreBindings.swift:105:9
    frame #21: 0x00000001018b6ec0 libllbuildSwift.dylib`BuildEngine.build(key=llbuildSwift.Key @ 0x000000016fdf36b8) at CoreBindings.swift:406:22
    frame #22: 0x0000000100087ef8 libSwiftDriverExecution.dylib`LLBuildEngine.build<SwiftDriverExecution.ExecuteAllJobsRule.RuleKey>(key=SwiftDriverExecution.ExecuteAllJobsRule.RuleKey @ 0xffffffffffffffff) at llbuild.swift:97:24
    frame #23: 0x000000010008f950 libSwiftDriverExecution.dylib`MultiJobExecutor.execute(env=29 key/value pairs, fileSystem=TSCBasic.LocalFileSystem @ 0x000000016fdf3c48) at MultiJobExecutor.swift:305:29
    frame #24: 0x000000010009b2a0 libSwiftDriverExecution.dylib`SwiftDriverExecutor.execute(workload=SwiftDriver.DriverExecutorWorkload @ 0x000000016fdf5170, delegate=0x0000000b84039b30, numParallelJobs=1, forceResponseFiles=false, recordedInputModificationDates=0 key/value pairs) at SwiftDriverExecutor.swift:88:25
    frame #26: 0x00000001007779d4 libSwiftDriver.dylib`Driver.performTheBuild(allJobs=2 values, jobExecutionDelegate=0x0000000b84039b30, forceResponseFiles=false) at Driver.swift:1908:18
    frame #27: 0x0000000100773e5c libSwiftDriver.dylib`Driver.run(jobs=2 values) at Driver.swift:1844:13
    frame #28: 0x00000001000044a8 swiftc`main at main.swift:171:14
    frame #29: 0x000000019a401e4c dyld`start + 6768