Fix transform narrowing warnings

Catch passes ::tolower into std::transform with string iterators.
::tolower has the signature int(int), which triggers a stealth narrowing
warning inside std::transform, because transform calls
*_Dest = _Fn(*_First), which implicitly narrows an int to a char.

For this particular application the narrowing is fine, so explicitly
narrow in a lambda.

include/internal/catch_common.hpp

@@ -22,7 +22,8 @@ namespace Catch {
         return s.find( infix ) != std::string::npos;
     }
     void toLowerInPlace( std::string& s ) {
-        std::transform( s.begin(), s.end(), s.begin(), ::tolower );
+        std::transform( s.begin(), s.end(), s.begin(),
+            [](char c) { return static_cast<char>(::tolower(c)); } );
     }
     std::string toLower( std::string const& s ) {
         std::string lc = s;