[lld] [llvm] Rename `F_no_mmap` to `F_mmap` (PR #134787)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Hi @chestnykh ,
> 
> I have reverted these changes in an attempt to get the `llvm-clang-x86_64-sie-win` Build Bot passing again. I did a small amount of investigation into the failure -- I am curious if the logic here:
> 
> https://github.com/llvm/llvm-project/blob/d6c8e8908d421979a428ede9f9a630dac8576c5b/llvm/lib/Support/FileOutputBuffer.cpp#L189-L192
> 
> needs to be flipped. If my understanding is correct, then I believe flags should also be set to 0 in
> 
> https://github.com/llvm/llvm-project/blob/d6c8e8908d421979a428ede9f9a630dac8576c5b/llvm/unittests/Support/FileOutputBufferTest.cpp#L126
> 
> Please let me know what you think.

--no-mmap-output-file is actually our default. This PR did not change the default, and actually switched from createOnDiskBuffer to createInMemoryBuffer.

I've sent a reland #139836 with an explanation.

@dgg5503 Can you investigate why `lld::link-open-file.test` failed with `createInMemoryBuffer`?
```
   return createInMemoryBuffer(Path, Size, Mode); 
```

https://github.com/llvm/llvm-project/pull/134787