Remove FOLLY_NORETURN from definition.

Summary:
- Remove FOLLY_NORETURN from function definitions, which was causing
compilation errors on some platforms.

Test Plan: - Compile on Mac OS X with Clang

Reviewed By: tudorb@fb.com

FB internal diff: D1111416

folly/detail/FunctionalExcept.cpp

@@ -20,15 +20,15 @@
 
 FOLLY_NAMESPACE_STD_BEGIN
 
-void __throw_length_error(const char* msg) FOLLY_NORETURN {
+void __throw_length_error(const char* msg) {
   throw std::length_error(msg);
 }
 
-void __throw_logic_error(const char* msg) FOLLY_NORETURN {
+void __throw_logic_error(const char* msg) {
   throw std::logic_error(msg);
 }
 
-void __throw_out_of_range(const char* msg) FOLLY_NORETURN {
+void __throw_out_of_range(const char* msg) {
   throw std::out_of_range(msg);
 }