Clang: Fix splitting flags

Correct syntax of using regex substitution. The missing
replacement seems to have resulted performing the search
case insensitive which in turn resulted in splitting within
an option. Fix this by explicitly passing an empty
replacement.

Change-Id: Ieccde1f9daae445d0ebe3b7454f56d09f0139e3c
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/shared/clang/clang_installation.pri

@@ -93,9 +93,9 @@ defineReplace(splitFlags) {
     for (flag, flags_temp) {
         equals(inside_quotes, 0) {
             inside_quotes = 1
-            flag ~= s,-I\S*,
-            flag ~= s,/D\S*,
-            flag ~= s,/Z\S*,
+            flag ~= s,-I\S*,,
+            flag ~= s,/D\S*,,
+            flag ~= s,/Z\S*,,
             result += $$split(flag, " ")
         } else {
             inside_quotes = 0