[BUG]: EpollBackend.h: error: type struct EpollBackend violates the C++ One Definition Rule [-Werror=odr]

Building with diagnostic flags enabled I encountered the following:

```
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/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:34: error: type ‘struct EpollBackend’ violates the C++ One Definition Rule [-Werror=odr]
34 | class EpollBackend : public EventBaseBackendBase {
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:34: note: a different type is defined in another translation unit
34 | class EpollBackend : public EventBaseBackendBase {
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:104: note: the first difference of corresponding definitions is field ‘events_’
104 | std::vector events_; // Cache allocation.
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:104: note: a field of same name but different type is defined in another translation unit
104 | std::vector events_; // Cache allocation.
/usr/lib/gcc/x86_64-pc-linux-gnu/14/include/g++-v14/bits/stl_vector.h:428: note: type name ‘std::vector >’ should match type name ‘std::vector >’
428 | class vector : protected _Vector_base<_Tp, _Alloc>
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:45: error: type of ‘__ct_comp ’ does not match original declaration [-Werror=lto-type-mismatch]
45 | explicit EpollBackend(Options options);
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.cpp:176: note: ‘__ct_comp ’ was previously declared here
176 | EpollBackend::EpollBackend(Options options) : options_(options) {
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.cpp:176: note: code may be misoptimized unless ‘-fno-strict-aliasing’ is used
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:50: error: type of ‘getPollableFd’ does not match original declaration [-Werror=lto-type-mismatch]
50 | int getPollableFd() const override { return epollFd_; }
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:50: note: ‘getPollableFd’ was previously declared here
50 | int getPollableFd() const override { return epollFd_; }
/var/tmp/portage/dev-cpp/folly-2025.02.17.00/work/folly-2025.02.17.00/folly/io/async/EpollBackend.h:50: note: code may be misoptimized unless ‘-fno-strict-aliasing’ is used
lto1: some warnings being treated as errors
lto-wrapper: fatal error: /usr/bin/x86_64-pc-linux-gnu-g++ returned 1 exit status
compilation terminated.
/usr/lib/gcc/x86_64-pc-linux-gnu/14/../../../../x86_64-pc-linux-gnu/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status

```

First reported for Gentoo on `2024.11.04.00`, reproduced on `2025.02.17.00`

[Build Log](https://github.com/user-attachments/files/18880427/folly-2025.02.17.00.20250220-033225.log)

See also: https://bugs.gentoo.org/949607

Start with folly/io/async/EpollBackend.h and folly/io/async/EpollBackend.cpp, then reproduce the CMake build using the diagnostic flags shown in the report. Trace the duplicate or inconsistent EpollBackend definition and confirm that the diagnostic build completes without the ODR error.

Written by the indexing model from the issue text.