Fix Vim E885: not possible to change sign

Clang will sometimes give a diagnostic for line 0, and Vim borks on
placing signs for line 0.

python/ycm/diagnostic_interface.py

@@ -45,7 +45,9 @@ class DiagnosticInterface( object ):
 
 
   def UpdateWithNewDiagnostics( self, diags ):
-    self._buffer_number_to_line_to_diags = _ConvertDiagListToDict( diags )
+    normalized_diags = [ _NormalizeDiagnostic( x ) for x in diags ]
+    self._buffer_number_to_line_to_diags = _ConvertDiagListToDict(
+        normalized_diags )
 
     if self._user_options[ 'enable_diagnostic_signs' ]:
       self._placed_signs, self._next_sign_id = _UpdateSigns(
@@ -58,7 +60,7 @@ class DiagnosticInterface( object ):
 
     if self._user_options[ 'always_populate_location_list' ]:
       vimsupport.SetLocationList(
-        vimsupport.ConvertDiagnosticsToQfList( diags ) )
+        vimsupport.ConvertDiagnosticsToQfList( normalized_diags ) )
 
 
   def _EchoDiagnosticForLine( self, line_num ):
@@ -204,6 +206,16 @@ def _DiagnosticIsError( diag ):
   return diag[ 'kind' ] == 'ERROR'
 
 
+def _NormalizeDiagnostic( diag ):
+  def ClampToOne( value ):
+    return value if value > 0 else 1
+
+  location = diag[ 'location' ]
+  location[ 'column_num' ] = ClampToOne( location[ 'column_num' ] )
+  location[ 'line_num' ] = ClampToOne( location[ 'line_num' ] )
+  return diag
+
+
 class _DiagSignPlacement( namedtuple( "_DiagSignPlacement",
                                       [ 'id', 'line', 'buffer', 'is_error' ] ) ):
   # We want two signs that have different ids but the same location to compare