Rwlock try upgrade

[ShrigmaDev]

Tracking issue: #138559

This PR adds a try_upgrade method on RwLockReadGuard that transforms it into a RwLockWriteGuard if it is immediately possible (ie if there are no read guards except for this one at this time)

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[ShrigmaDev]

Not sure how to proceed in rwlock.rs, since the RwLock variables must be sized, like the compiler error suggests. Basically I'm unsure how to convert a RwLockReadGuard into a RwLockWriteGuard. Do you have any pointers @thomcc ?

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#19 exporting to docker image format
#19 sending tarball 19.7s done
#19 DONE 25.1s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'build.print-step-timings', '--enable-verbose-tests', '--set', 'build.metrics', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: build.build          := x86_64-unknown-linux-gnu
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
     |
858  | impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     |          - this type parameter needs to be `Sized`
...
925  |             data: UnsafeCell::new(orig.data.as_ptr().read()),
     |                                                      ^^^^ doesn't have a size known at compile-time
     |
note: required by a bound in `core::ptr::mut_ptr::<impl *mut T>::read`
    --> /checkout/library/core/src/ptr/mut_ptr.rs:1353:12
     |
---
858  + impl<'a, T> RwLockReadGuard<'a, T> {
     |
help: consider removing this method call, as the receiver has type `NonNull<T>` and `NonNull<T>: core::marker::Sized` trivially holds
     |
925  -             data: UnsafeCell::new(orig.data.as_ptr().read()),
925  +             data: UnsafeCell::new(orig.data.read()),
     |

error[E0277]: the size for values of type `T` cannot be known at compilation time
    --> library/std/src/sync/poison/rwlock.rs:925:35
     |
858  | impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     |          - this type parameter needs to be `Sized`
...
925  |             data: UnsafeCell::new(orig.data.as_ptr().read()),
     |                   --------------- ^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
     |                   |
     |                   required by a bound introduced by this call
     |
note: required by a bound in `UnsafeCell::<T>::new`
---
     |
858  - impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
858  + impl<'a, T> RwLockReadGuard<'a, T> {
     |
help: consider removing this method call, as the receiver has type `*mut T` and `*mut T: core::marker::Sized` trivially holds
     |
925  -             data: UnsafeCell::new(orig.data.as_ptr().read()),
925  +             data: UnsafeCell::new(orig.data.as_ptr()),
     |

error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> library/std/src/sync/poison/rwlock.rs:924:20
    |
858 |   impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
    |            - this type parameter needs to be `Sized`
...
924 |           let rwl = &RwLock {
    |  ____________________^
925 | |             data: UnsafeCell::new(orig.data.as_ptr().read()),
926 | |             poison: poison::Flag::new(),
927 | |             inner: *orig.inner_lock,
928 | |         };
    | |_________^ doesn't have a size known at compile-time
    |
note: required because it appears within the type `poison::rwlock::RwLock<T>`
   --> library/std/src/sync/poison/rwlock.rs:80:12