[libc++] Make <experimental/filesystem> compile with gcc 4.8.5

[vitaut]

Summary

This fixes an error when compiling <experimental/filesystem> with gcc 4.8.5:

.../libcxx/src/experimental/filesystem/filesystem_common.h:137:34:
error: redeclaration ‘T
std::experimental::filesystem::v1::detail::{anonymous}::error_value() [with T =
bool]’ d
iffers in ‘constexpr’
 constexpr bool error_value<bool>() {
                                  ^
.../libcxx/src/experimental/filesystem/filesystem_common.h:133:3:
error: from previous declaration ‘T
std::experimental::filesystem::v1::detail::{anonymous}::error_value() [with T
 = bool]’
 T error_value();
   ^

[EricWF]

Urg. That's an old compiler. But this looks OK to me.

[vitaut]

@EricWF could you or someone else commit this as I don't have commit access? Thanks.

[EricWF]

r337962.

Thanks.

src/experimental/filesystem/filesystem_common.h

error_code capture_errno() {
   return error_code(errno, generic_category());
 }
 
 template <class T>
 T error_value();
 template <>
 _LIBCPP_CONSTEXPR_AFTER_CXX11 void error_value<void>() {}
 template <>
-constexpr bool error_value<bool>() {
+bool error_value<bool>() {
   return false;
 }
 template <>
-constexpr uintmax_t error_value<uintmax_t>() {
+uintmax_t error_value<uintmax_t>() {
   return uintmax_t(-1);
 }
 template <>
 _LIBCPP_CONSTEXPR_AFTER_CXX11 file_time_type error_value<file_time_type>() {
   return file_time_type::min();
 }
 template <>
 path error_value<path>() {