libcpp: Fix up pragma preprocessing [PR100450]

Since the r0-85991-ga25a8f3be322fe0f838947b679f73d6efc2a412c
https://gcc.gnu.org/legacy-ml/gcc-patches/2008-02/msg01329.html
changes, so that we handle macros inside of pragmas that should expand
macros, during preprocessing we print those pragmas token by token,
with CPP_PRAGMA printed as
      fputs ("#pragma ", print.outf);
      if (space)
        fprintf (print.outf, "%s %s", space, name);
      else
        fprintf (print.outf, "%s", name);
where name is some identifier (so e.g. print
 #pragma omp parallel
or
 #pragma omp for
etc.).  Because it ends in an identifier, we need to handle it like
an identifier (i.e. CPP_NAME) for the decision whether a space needs
to be emitted in between that #pragma whatever or #pragma whatever whatever
and following token, otherwise the attached testcase is preprocessed as
 #pragma omp forreduction(+:red)
rather than
 #pragma omp for reduction(+:red)
The cpp_avoid_paste function is only called for this purpose.

2021-05-07  Jakub Jelinek  <jakub@redhat.com>

	PR c/100450
	* lex.c (cpp_avoid_paste): Handle token1 CPP_PRAGMA like CPP_NAME.

	* c-c++-common/gomp/pr100450.c: New test.

(cherry picked from commit 170c850e4b)

gcc/testsuite/c-c++-common/gomp/pr100450.c

@@ -0,0 +1,20 @@
+/* PR c/100450 */
+/* { dg-do compile } */
+/* { dg-options "-fopenmp -save-temps -Wunknown-pragmas" } */
+
+#define TEST(T) { \
+     {T} \
+}
+#define CLAUSES reduction(+:red)
+#define PARALLEL_FOR(X) TEST({ \
+_Pragma("omp for CLAUSES") \
+X \
+})
+
+void foo()
+{
+  int red = 0;
+  int A[3] = {};
+  #pragma omp parallel shared(red)
+  PARALLEL_FOR( for(int i=0; i < 3; i++) red += A[i]; )
+}

libcpp/lex.c

@@ -3485,6 +3485,7 @@ cpp_avoid_paste (cpp_reader *pfile, const cpp_token *token1,
     case CPP_DEREF:	return c == '*';
     case CPP_DOT:	return c == '.' || c == '%' || b == CPP_NUMBER;
     case CPP_HASH:	return c == '#' || c == '%'; /* Digraph form.  */
+    case CPP_PRAGMA:
     case CPP_NAME:	return ((b == CPP_NUMBER
 				 && name_p (pfile, &token2->val.str))
 				|| b == CPP_NAME