Revert "Make -Wstring-plus-int warns even if when the result is not out of bounds"

This reverts commit r349054.

It's causing:

FAILED: tools/clang/bindings/python/tests/CMakeFiles/check-clang-python
FAIL: test_diagnostic_range (tests.cindex.test_diagnostics.TestDiagnostics)
----------------------------------------------------------------------
Traceback (most recent call last):
  File
  "/Users/buildslave/jenkins/workspace/clang-stage1-configure-RA/llvm/tools/clang/bindings/python/tests/cindex/test_diagnostics.py",
  line 55, in test_diagnostic_range
      self.assertEqual(len(tu.diagnostics), 1)
      AssertionError: 2 != 1

======================================================================
FAIL: test_diagnostic_warning (tests.cindex.test_diagnostics.TestDiagnostics)
----------------------------------------------------------------------
Traceback (most recent call last):
  File
  "/Users/buildslave/jenkins/workspace/clang-stage1-configure-RA/llvm/tools/clang/bindings/python/tests/cindex/test_diagnostics.py",
  line 18, in test_diagnostic_warning
      self.assertEqual(len(tu.diagnostics), 2)
      AssertionError: 1 != 2

llvm-svn: 349117