errors: indent "result type clash" error context

This used to be the format of the error report:

  input.y:6.5-10: result type clash on merge function 'merge': [...]
  input.y:2.4-9: previous declaration

In order to distinguish the actual error from the context provided, we
rather this new output:

  input.y:6.5-10: result type clash on merge function 'merge': [...]
  input.y:2.4-9:     previous declaration

Another patch will introduce an "error: " prefix to all non-indented
lines, giving yet better readability to the reports.

* src/complain.h (SUB_INDENT): Move to here.
* src/reader.c (record_merge_function_type): Use complain_at_indent to
output with increased indentation level.
* src/scan-code.l (SUB_INDENT): Remove from here.
* tests/glr-regression.at: Apply this change.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>

src/complain.h

@@ -25,6 +25,9 @@
 extern "C" {
 # endif
 
+/* Sub-messages indent. */
+#define SUB_INDENT (4)
+
 /** Record that a warning is about to be issued, and treat it as an
     error if <tt>warnings_flag & warnings_error</tt>.  This is exported
     only for the sake of Yacc-compatible conflict reports in conflicts.c.