Utils: Add CommandLine convenience constructors

... taking a QString for the executable.

This weakens the very explicit QString -> FileName conversion via the
named constructors for the special case of constructing a CommandLine.

I think that's worthwhile here, as it reduces the noise on the caller
site under circumstance where the nature of the thing is obvious.

Change-Id: I27b4a73639728893d053b2e7ba65cb745f0ffe83
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/valgrind/valgrindtestrunnertest.cpp

@@ -85,8 +85,7 @@ QString ValgrindTestRunnerTest::runTestBinary(const QString &binary, const QStri
     debuggee.executable = Utils::FilePath::fromString(binPath);
     debuggee.environment = Utils::Environment::systemEnvironment();
 
-    CommandLine valgrind{FilePath::fromString("valgrind")};
-    valgrind.addArgs({"--num-callers=50", "--track-origins=yes"});
+    CommandLine valgrind{"valgrind", {"--num-callers=50", "--track-origins=yes"}};
     valgrind.addArgs(vArgs);
 
     m_runner->setLocalServerAddress(QHostAddress::LocalHost);