[test] Fix validation-test/Driver/many-inputs.swift differently.

It turns out the problem was with the full length of the argument list
in characters (bytes), not just the /number/ of arguments. Defeat this
by using paths relative to the temporary directory we're using.

rdar://problem/25617239, hopefully for the last time.

Author: jrose-apple

validation-test/Driver/many-inputs.swift

@@ -4,17 +4,15 @@
 // at one point.
 // RUN: for i in {1..1100}; do echo "public func foo$i() {}" > %t/$i.swift; echo "CHECK: foo$i" >> %t/check.txt; done
 
-// We're not using %target-swift-build because we don't want to run into any
-// limits *xcrun* has on arguments.
-// RUN: %swiftc_driver -sdk %sdk -target %target-triple -force-single-frontend-invocation -emit-library %t/*.swift -o %t/libWMO
+// RUN: (cd %t && %target-build-swift -force-single-frontend-invocation -emit-library ./*.swift -o ./libWMO)
 // RUN: nm %t/libWMO | FileCheck %t/check.txt
 
-// RUN: %swiftc_driver -sdk %sdk -target %target-triple -force-single-frontend-invocation -num-threads 1 -emit-library %t/*.swift -o %t/libWMOThreaded
+// RUN: (cd %t && %target-build-swift -force-single-frontend-invocation -num-threads 1 -emit-library ./*.swift -o ./libWMOThreaded)
 // RUN: nm %t/libWMOThreaded | FileCheck %t/check.txt
 
 // This is very slow due to process overhead. It's also doing one file at a time
 // because we don't have a good way for lit tests to claim more than one thread.
 // But it's still important to check.
-// RUN: %swiftc_driver -sdk %sdk -target %target-triple -emit-library %t/*.swift -o %t/libMultiFile
+// RUN: (cd %t && %target-build-swift -emit-library ./*.swift -o ./libMultiFile)
 // RUN: nm %t/libMultiFile | FileCheck %t/check.txt