FPEnv.h - reduce includes to forward declarations. NFC.

Ensure FPEnv.cpp includes FPEnv.h first to check for hidden dependencies.
diff --git a/llvm/lib/IR/FPEnv.cpp b/llvm/lib/IR/FPEnv.cpp
index 40f999e..910ec98 100644
--- a/llvm/lib/IR/FPEnv.cpp
+++ b/llvm/lib/IR/FPEnv.cpp
@@ -12,8 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/ADT/StringSwitch.h"
 #include "llvm/IR/FPEnv.h"
+#include "llvm/ADT/StringSwitch.h"
 
 namespace llvm {