qt5: add upstream patch for 10.11

Technically: Add upstream patch for the Clang that ships with OS X 10.11. If
there's a demand, we'll likely backport this to 10.10 as well for Xcode-Beta users.

References:
* https://bugreports.qt.io/browse/QTBUG-47383
* https://codereview.qt-project.org/#/c/121545/

Reference: #40837.
Closes #42014.

Library/Formula/qt5.rb

@@ -29,6 +29,16 @@ class Qt5 < Formula
     # This is fixed in 5.5 branch and below patch should be removed
     # when this formula is updated to 5.5.1
     patch :DATA
+
+    # Upstream commit to fix the fatal build error on OS X El Capitan.
+    # https://codereview.qt-project.org/#/c/121545/
+    # Should land in the 5.5.1 release.
+    if MacOS.version >= :el_capitan
+      patch do
+        url "https://raw.githubusercontent.com/DomT4/scripts/2107043e8/Homebrew_Resources/Qt5/qt5_el_capitan.diff"
+        sha256 "bd8fd054247ec730f60778e210d58cba613265e5df04ec93f4110421fb03b14a"
+      end
+    end
   end
 
   bottle do