CC Mode. Preserve space syntax-table props in open strings

CC Mode.  Preserve space syntax-table props in open strings

This fixes bug#68111, allowing, e.g., backward-kill-word to
work properly.

* lisp/progmodes/cc-mode.el (c-clear-string-fences): On an open
string which isn't followed by another string, remove the
syntax-table properties just from template delimiters rather
than setting the syntax-table properties of all the string to
punctuation.