Use "not" instead of "set +o pipefail"; improve "broken" header.

A header shouldn't contain a function body unless the function is
inline. Following gribozavr's suggestion, I'm using the easier solution
of omitting the semicolon from the function prototype.

(In response to review comments.)

Author: martinboehme

test/Frontend/Inputs/broken-c-module/broken_c.h

@@ -1,3 +1,2 @@
-// This C code intentionally doesn't compile (note missing parenthesis).
-void foo(int bar {
-}
+// This C code intentionally doesn't compile (note missing semicolon).
+void foo()

test/Frontend/verify-broken-c-module.swift

@@ -3,12 +3,9 @@
 // This needs to be a separate test from verify.swift because compilation will
 // terminate after the failing import statement.
 
-// Turn off "pipefail" mode because we intend the `swift` invocation to
-// terminate with a non-zero exit code, and we don't want the test to fail
-// because of this.
-// RUN: set +o pipefail && %target-typecheck-verify-swift -I %S/Inputs/broken-c-module 2>&1 | %FileCheck %s
+// RUN: not %target-typecheck-verify-swift -I %S/Inputs/broken-c-module 2>&1 | %FileCheck %s
 
 // CHECK: [[@LINE+3]]:8: error: unexpected error produced: could not build C module 
 // CHECK: note: in file included from <module-includes>
-// CHECK: broken_c.h:2:18: error: expected ')'
+// CHECK: broken_c.h:2:11: error: expected function body after function declarator
 import BrokenCModule

test/Frontend/verify.swift

@@ -1,9 +1,6 @@
 // Tests for the Swift frontends `-verify` mode.
 
-// Turn off "pipefail" mode because we intend the `swift` invocation to
-// terminate with a non-zero exit code, and we don't want the test to fail
-// because of this.
-// RUN: set +o pipefail && %target-typecheck-verify-swift 2>&1 | %FileCheck %s
+// RUN: not %target-typecheck-verify-swift 2>&1 | %FileCheck %s
 
 // CHECK: [[@LINE+1]]:1: error: unexpected error produced: use of unresolved
 undefinedFunc()