Fix compilation for libc++ 3.9

Summary:
libc++ 3.9 and above define the __throw* functions inside stdexcept, so
defining them ourselves leads to compilation errors when compiling
against libc++ 3.9. Add appropriate preprocessor guards to avoid this.

Reviewed By: meyering

Differential Revision: D3898284

fbshipit-source-id: 435a28c2b3a83ee4d8f5af0df0343c524469011e

folly/portability/BitsFunctexcept.cpp

@@ -21,6 +21,7 @@
 
 FOLLY_NAMESPACE_STD_BEGIN
 
+#if !defined(_LIBCPP_VERSION) || _LIBCPP_VERSION < 3900
 void __throw_length_error(const char* msg) {
   throw std::length_error(msg);
 }
@@ -32,6 +33,7 @@ void __throw_logic_error(const char* msg) {
 void __throw_out_of_range(const char* msg) {
   throw std::out_of_range(msg);
 }
+#endif
 
 #ifdef _MSC_VER
 void __throw_bad_alloc() {

folly/portability/BitsFunctexcept.h

@@ -25,9 +25,11 @@
 #include <folly/Portability.h>
 FOLLY_NAMESPACE_STD_BEGIN
 
+#if !defined(_LIBCPP_VERSION) || _LIBCPP_VERSION < 3900
 [[noreturn]] void __throw_length_error(const char* msg);
 [[noreturn]] void __throw_logic_error(const char* msg);
 [[noreturn]] void __throw_out_of_range(const char* msg);
+#endif
 
 #ifdef _MSC_VER
 [[noreturn]] void __throw_bad_alloc();