libcpp: Fix paste error with unknown pragma after macro expansion

In directives.cc, do_pragma() contains logic to handle a case such as the new
testcase pragma-omp-unknown.c, where an unknown pragma was the result of macro
expansion (for pragma namespaces that permit expansion). This no longer works
correctly as shown by the testcase, fixed by adding PREV_WHITE to the flags on
the second token to prevent an unwanted paste.  Also fixed the memory leak,
since the temporary tokens are pushed on their own context, nothing prevents
freeing of the buffer that holds them when the context is eventually popped.

libcpp/ChangeLog:

	* directives.cc (do_pragma): Fix memory leak in token buffer.  Fix
	unwanted paste between two tokens.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/pragma-omp-unknown.c: New test.