This was previously being appended to the output string *after* a copy
of the supposedly final string was made and handed to the caller. So
the diagnostic was never actually visible to the user.
We fix this by moving the check for an unterminated #if from
glcpp_parser_destroy to the calling function, preprocess.
This fixes the test case 083-unterminated-if.c.
Rather than telling the user what to fix, the standard convention is to
describe what the detected problem is. With this change, test
081-elif-without-expression now passes.
This is now consistent with other usage of flex/bison througout mesa,
(which is that these generated files are added to source control so
that the build system does not require external tools like flex/bison
for non-developers).
