[test] Redirect stderr to a different file to avoid mixing with stdout.

In Windows stdout and stderr are not line buffered, so the output of
stderr can be intermixed with the stdout, which will break the test when
checking with CHECK-NEXT and similar. Additionally it seems that stderr
always contains at least one new line.

Redirecting stderr to a file removes the problems with the mixing, and
since the stderr is not checked for this test, should not change
anything.

Author: drodriguez

test/Frontend/ast-dump.swift

@@ -4,9 +4,9 @@
 // RUN: echo 'public func main() {a(); b()}' >%t/main.swift
 
 // Test printing to stdout
-// RUN: %target-swift-frontend -dump-ast -primary-file %t/a.swift %t/b.swift %t/main.swift -module-name main -o - 2>&1 | %FileCheck -check-prefix A-AST %s
-// RUN: %target-swift-frontend -dump-ast %t/a.swift -primary-file %t/b.swift %t/main.swift -module-name main -o - 2>&1 | %FileCheck -check-prefix B-AST %s
-// RUN: %target-swift-frontend -dump-ast %t/a.swift %t/b.swift -primary-file %t/main.swift -module-name main -o - 2>&1 | %FileCheck -check-prefix MAIN-AST %s
+// RUN: %target-swift-frontend -dump-ast -primary-file %t/a.swift %t/b.swift %t/main.swift -module-name main -o - 2>%t/a.swift.stderr | %FileCheck -check-prefix A-AST %s
+// RUN: %target-swift-frontend -dump-ast %t/a.swift -primary-file %t/b.swift %t/main.swift -module-name main -o - 2>%t/b.swift.stderr | %FileCheck -check-prefix B-AST %s
+// RUN: %target-swift-frontend -dump-ast %t/a.swift %t/b.swift -primary-file %t/main.swift -module-name main -o - 2>%t/main.swift.stderr | %FileCheck -check-prefix MAIN-AST %s
 
 // Test printing to files
 // RUN: %target-swift-frontend -dump-ast -primary-file %t/a.swift %t/b.swift %t/main.swift -module-name main -o %t/a.ast