Fix dejagnu pruning of constexpr context messages.

I wonder why nobody fixed this until now; it's been two years since the
diagnostic context output was changed.

	* lib/prune.exp (prune_gcc_output): Adjust constexpr pattern.

gcc/testsuite/ChangeLog

+2020-01-14  Jason Merrill  <jason@redhat.com>
+
+	* lib/prune.exp (prune_gcc_output): Adjust constexpr pattern.
+
 2020-01-14  Xiong Hu Luo  <luoxhu@linux.ibm.com>
 
 	PR ipa/69678

gcc/testsuite/lib/prune.exp

@@ -35,7 +35,7 @@ proc prune_gcc_output { text } {
     regsub -all "(^|\n)\[^\n\]*(: )?At (top level|global scope):\[^\n\]*" $text "" text
     regsub -all "(^|\n)\[^\n\]*:   (recursively )?required \[^\n\]*" $text "" text
     regsub -all "(^|\n)\[^\n\]*:   . skipping \[0-9\]* instantiation contexts \[^\n\]*" $text "" text
-    regsub -all "(^|\n)\[^\n\]*:   in constexpr expansion \[^\n\]*" $text "" text
+    regsub -all "(^|\n)\[^\n\]*:   in .constexpr. expansion \[^\n\]*" $text "" text
     regsub -all "(^|\n)\[^\n\]*:   in requirements \[^\n\]*" $text "" text
     regsub -all "(^|\n)    inlined from \[^\n\]*" $text "" text
     regsub -all "(^|\n)collect2: error: ld returned \[^\n\]*" $text "" text