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>

THANKS

@@ -102,6 +102,7 @@ Shura                     debil_urod@ngs.ru
 Stefano Lattarini         stefano.lattarini@gmail.com
 Steve Murphy              murf@parsetree.com
 Sum Wu                    sum@geekhouse.org
+Théophile Ranquet         theophile.ranquet@gmail.com
 Thiru Ramakrishnan        thiru.ramakrishnan@gmail.com
 Tim Josling               tej@melbpc.org.au
 Tim Landscheidt           tim@tim-landscheidt.de
@@ -125,6 +126,7 @@ thank them!  Please, help us keeping this list up to date.
 
 Local Variables:
 mode: text
+coding: utf-8
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