[utils] fix off-by-one line number bug

When folding up lines of nested expansions, the line number would be off
by 1, which would lead to lines being shuffled around.

Author: hnrklssn

test/Utils/update-verify-tests/expansion.swift

@@ -132,9 +132,9 @@ func foo(_ x: Int) {
 }
 """)
   //expected-expansion@+5:14{{
-    //   expected-error@3  {{cannot find 'b' in scope; did you mean 'x'?}}
     //   expected-note@1   2{{'x' declared here}}
     //   expected-error@2   {{cannot find 'a' in scope; did you mean 'x'?}}
+    //   expected-error@3  {{cannot find 'b' in scope; did you mean 'x'?}}
   //}}
 func foo() {}
 

utils/update_verify_tests/core.py

@@ -262,6 +262,7 @@ def parse_diag(line, filename, prefix):
 
 
 def add_line(new_line, lines):
+    assert new_line.line_n > 0
     lines.insert(new_line.line_n - 1, new_line)
     for i in range(new_line.line_n, len(lines)):
         line = lines[i]
@@ -429,7 +430,7 @@ def fold_expansions(lines):
         if line.diag.category == "closing":
             line.diag.parent.closer = line
         else:
-            line.line_n = len(line.diag.parent.nested_lines)
+            line.line_n = len(line.diag.parent.nested_lines) + 1
             add_line(line, line.diag.parent.nested_lines)