[PATCH] [Support][Endian] Overload += and -=

[David Majnemer]

LGTM with tweaks.

================
Comment at: include/llvm/Support/Endian.h:96
@@ -95,1 +95,3 @@
 
+  packed_endian_specific_integral& operator+=(value_type newValue) {
+    *this = *this + newValue;
----------------
Move the `&` next to the `operator` keyword.

================
Comment at: include/llvm/Support/Endian.h:101
@@ +100,3 @@
+
+  packed_endian_specific_integral& operator-=(value_type newValue) {
+    *this = *this - newValue;
----------------
Ditto.

http://reviews.llvm.org/D5107