Mark qt_from_latin1() as nothrow

It's used from a different TU, too, so we can't rely on the
compiler to inspect it to draw this conclusion itself.

This function has technically not a wide contract, because
the output buffer needs to be large enough to hold the result.
However, this precondition cannot be checked from within the
function, therefore no assertion can ever be added and the
nothrow marker becomes acceptable (even desireable).

Change-Id: I2dc6c4f3d9d8147c6483865c5c4bbc8e9af291b7
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/codecs/qtextcodec.cpp

@@ -993,7 +993,7 @@ QString QTextDecoder::toUnicode(const char *chars, int len)
 }
 
 // in qstring.cpp:
-void qt_from_latin1(ushort *dst, const char *str, size_t size);
+void qt_from_latin1(ushort *dst, const char *str, size_t size) Q_DECL_NOTHROW;
 
 /*! \overload
 

src/corelib/tools/qstring.cpp

@@ -203,7 +203,7 @@ inline RetType UnrollTailLoop<0>::exec(int, RetType returnIfExited, Functor1, Fu
 #endif
 
 // conversion between Latin 1 and UTF-16
-void qt_from_latin1(ushort *dst, const char *str, size_t size)
+void qt_from_latin1(ushort *dst, const char *str, size_t size) Q_DECL_NOTHROW
 {
     /* SIMD:
      * Unpacking with SSE has been shown to improve performance on recent CPUs