[libc] [llvm] [libc][NFC] Refactor FPBits and remove LongDoubleBits specialization (PR #78192)

[Clement Courbet via llvm-commits]

================
@@ -220,6 +270,9 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
   }
 
   LIBC_INLINE constexpr StorageType uintval() const { return bits & FP_MASK; }
+  LIBC_INLINE constexpr StorageType set_uintval(StorageType value) {
+    return bits = (value & FP_MASK);
----------------
legrosbuffle wrote:

```
bits = (value & FP_MASK);
return bits;
```

https://github.com/llvm/llvm-project/pull/78192