Make <experimental/filesystem> compile with gcc 4.8.5

Patch by Victor Zverovich.

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();
   ^
```

Reviewed as https://reviews.llvm.org/D49813

llvm-svn: 337962