SSO strings: bugfix (#25810)

tests/errmsgs/tsso_string_index_var.nim

@@ -0,0 +1,13 @@
+discard """
+  cmd: "nim check --strings:sso --mm:orc --hints:off $file"
+  action: "reject"
+  nimout: '''
+tsso_string_index_var.nim(13, 12) Error: expression 's[0]' is immutable, not 'var'
+'''
+"""
+
+proc passByVar(c: var char) =
+  c = 'x'
+
+var s = "abc"
+passByVar(s[0])