Fix typo “a Rc” → “an Rc” (and a few more)

[steffahn]

After stumbling about it in the dev-guide, I’ve devided to eliminate all mentions of “a Rc”, replacing it with “an Rc”. E.g.

$ rg "(^|[^'])\ba\b[^\w=:]*\bRc" 
compiler/rustc_data_structures/src/owning_ref/mod.rs
1149:/// Typedef of a owning reference that uses a `Rc` as the owner.

library/std/src/ffi/os_str.rs
919:    /// Converts a [`OsString`] into a [`Rc`]`<OsStr>` without copying or allocating.

library/std/src/ffi/c_str.rs
961:    /// Converts a [`CString`] into a [`Rc`]`<CStr>` without copying or allocating.

src/doc/rustc-dev-guide/src/query.md
61:are cheaply cloneable; insert a `Rc` if necessary).

src/doc/book/src/ch15-06-reference-cycles.md
72:decreases the reference count of the `a` `Rc<List>` instance from 2 to 1 as

library/alloc/src/rc.rs
1746:    /// Converts a generic type `T` into a `Rc<T>`

(the match in the book is a false positive)
Since the dev-guide is a submodule, it’s getting a separate PR: rust-lang/rustc-dev-guide#1191

I’ve also gone ahead and done the same search for RwLock and hit a few cases in the OwningRef adaption. Then, I couldn’t keep the countless cases of “a owning …” or “a owner” unaddressed, which concludes this PR.

@rustbot label C-cleanup

[rust-highfive]

r? @michaelwoerister

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[bors]

☔ The latest upstream changes (presumably #88265) made this pull request unmergeable. Please resolve the merge conflicts.

[steffahn]

Unsurprisingly, I’ve created my own merge conflict here…

[steffahn]

…now resolved ^^

[michaelwoerister]

Thanks, @steffahn!

@bors r+ rollup

[bors]

📌 Commit 6248dbc has been approved by michaelwoerister