QLibraryInfo: mark isDebugBuild() noexcept and CONST

It's literally

   return true; // or
   return false;

so it can be marked as noexcept and (the stronger form of) pure.

Change-Id: Id55fee1db5d18201be2ea6c6778ece3de68b05f2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qlibraryinfo.cpp

@@ -291,7 +291,7 @@ const char *QLibraryInfo::build() noexcept
     false if it was built in release mode.
 */
 bool
-QLibraryInfo::isDebugBuild()
+QLibraryInfo::isDebugBuild() noexcept
 {
 #ifdef QT_DEBUG
     return true;

src/corelib/global/qlibraryinfo.h

@@ -51,7 +51,7 @@ class Q_CORE_EXPORT QLibraryInfo
 public:
     static const char *build() noexcept;
 
-    static bool isDebugBuild();
+    static bool isDebugBuild() noexcept Q_DECL_CONST_FUNCTION;
 
 #ifndef QT_BOOTSTRAPPED
     static QVersionNumber version() noexcept Q_DECL_CONST_FUNCTION;