Rollup merge of #63036 - topecongiro:add-lib-section, r=matklad

Add lib section to rustc_lexer's Cargo.toml

This is required to fix the rustc-ap-syntax build error in the recent version. The error could also be fixed on the [rustc-auto-publish](https://github.com/alexcrichton/rustc-auto-publish) side by manually adding `[lib]` section if one does not exist. The latter approach, however, may have a surprising side effect, so I am opting for a simpler solution for now.

r? @alexcrichton

src/librustc_lexer/Cargo.toml

@@ -7,3 +7,9 @@ edition = "2018"
 # Note that this crate purposefully does not depend on other rustc crates
 [dependencies]
 unicode-xid = { version = "0.1.0", optional = true }
+
+# Note: do not remove this blank `[lib]` section.
+# This will be used when publishing this crate as `rustc-ap-rustc_lexer`.
+[lib]
+doctest = false
+name = "rustc_lexer"