testsuite: Provide means to regexp in multiline patterns

Those multi-line-patterns are literal.  Sometimes a regexp
needs to be matched.  This is a start: just three elements
are supported: "(" ")" and the compound ")?" (and on second
thought, it can be argued that "(...)" alone is not useful).
Note that Tcl "string map" is documented to have the desired
effect: a once-over but no re-recognitions of previously
replaced mapped elements.  Also, drop a doubled "containing".

testsuite:
	* lib/multiline.exp (_build_multiline_regex): Map
	"{re:" to "(", similarly ")?" from ":re?}" and the
	same without question mark.

gcc/testsuite/lib/multiline.exp

@@ -297,7 +297,7 @@ proc _get_lines { filename first_line last_line } {
 
 # Convert $multiline from a list of strings to a multiline regex
 # We need to support matching arbitrary followup text on each line,
-# to deal with comments containing containing DejaGnu directives.
+# to deal with comments containing DejaGnu directives.
 
 proc _build_multiline_regex { multiline index } {
     verbose "_build_multiline_regex: $multiline $index" 4
@@ -307,7 +307,10 @@ proc _build_multiline_regex { multiline index } {
 	verbose "  line: $line" 4
 
 	# We need to escape "^" and other regexp metacharacters.
-	set line [string map {"^" "\\^"
+	set line [string map {"\{re:" "("
+	                      ":re?\}" ")?"
+	                      ":re\}" ")"
+	                      "^" "\\^"
 	                      "(" "\\("
 	                      ")" "\\)"
 	                      "[" "\\["