c++: Make C++11 raw string recognition stricter
Reported by Vaclav Slavik at:
<http://savannah.gnu.org/bugs/?43970>.
* gettext-tools/src/x-c.c (struct token_ty): New field 'escape'.
(struct xgettext_token_ty): New field 'escape'.
(phase5_get): Recognize raw strings more strictly. Set 'escape'
field of token appropriately for string literals.
(extract_parenthesized): Respect 'escape' field of token.
* gettext-tools/tests/xgettext-c-20: Adjust to the latest xgettext C++
scanner change; don't accept unbalanced prefix in raw string literal.
