/usr/src/llvm-2.9/patches/0011-Hurd-fixes.patch is in llvm-2.9-source 2.9+dfsg-3ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 | ---
 Path.inc    |    5 +++++
 Process.inc |    4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)
--- a/lib/Support/Unix/Path.inc
+++ b/lib/Support/Unix/Path.inc
@@ -64,6 +64,11 @@
 #include <unistd.h>
 #endif
 
+// For GNU Hurd
+#if defined(__GNU__) && !defined(MAXPATHLEN)
+# define MAXPATHLEN 4096
+#endif
+
 // Put in a hack for Cygwin which falsely reports that the mkdtemp function
 // is available when it is not.
 #ifdef __CYGWIN__
--- a/lib/Support/Unix/Process.inc
+++ b/lib/Support/Unix/Process.inc
@@ -136,7 +136,7 @@
   return getgid();
 }
 
-#ifdef HAVE_MACH_MACH_H
+#if defined(HAVE_MACH_MACH_H) && !defined(__GNU__)
 #include <mach/mach.h>
 #endif
 
@@ -150,7 +150,7 @@
   setrlimit(RLIMIT_CORE, &rlim);
 #endif
 
-#ifdef HAVE_MACH_MACH_H
+#if defined(HAVE_MACH_MACH_H) && !defined(__GNU__)
   // Disable crash reporting on Mac OS X 10.0-10.4
 
   // get information about the original set of exception ports for the task
 |