Xcode: Also quote strings containing // (#15487)

Otherwise those will be interpreted as start of a comment

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>

Tests/RunCMake/XcodeProject/XcodeObjectNeedsQuote-check.cmake

@@ -0,0 +1,7 @@
+set(expect "path = \"")
+file(STRINGS ${RunCMake_TEST_BINARY_DIR}/XcodeObjectNeedsQuote.xcodeproj/project.pbxproj actual
+     REGEX "path = [^;]*someFileWithoutSpecialChars[^;]*;" LIMIT_COUNT 1)
+if(NOT "${actual}" MATCHES "${expect}")
+  message(SEND_ERROR "The actual project contains the line:\n ${actual}\n"
+    "which does not match expected regex:\n ${expect}\n")
+endif()