Auto merge of rust-lang#51196 - Havvy:remove-keywords, r=petrochenkov

Implement RFC 2421, 'Keyword unreservations (pure, sizeof, alignof, offsetof)

On my local machine I was getting failures in `test/ui-fulldeps`, but I was also getting them again after reverting the change, so I'm hoping that this works.

I removed the test because that's what the other PR that unreserved a keyword did and I feel that it doesn't make sense/keep value to keep a test for something removed years ago.

.gitmodules

@@ -1,7 +1,3 @@
-[submodule "src/llvm"]
-	path = src/llvm
-	url = https://github.com/rust-lang/llvm.git
-	branch = master
 [submodule "src/jemalloc"]
 	path = src/jemalloc
 	url = https://github.com/rust-lang/jemalloc.git
@@ -11,39 +7,9 @@
 [submodule "src/liblibc"]
 	path = src/liblibc
 	url = https://github.com/rust-lang/libc.git
-[submodule "src/doc/nomicon"]
-	path = src/doc/nomicon
-	url = https://github.com/rust-lang-nursery/nomicon.git
-[submodule "src/tools/cargo"]
-	path = src/tools/cargo
-	url = https://github.com/rust-lang/cargo.git
-[submodule "src/doc/reference"]
-	path = src/doc/reference
-	url = https://github.com/rust-lang-nursery/reference.git
-[submodule "src/doc/book"]
-	path = src/doc/book
-	url = https://github.com/rust-lang/book.git
-[submodule "src/tools/rls"]
-	path = src/tools/rls
-	url = https://github.com/rust-lang-nursery/rls.git
-[submodule "src/libcompiler_builtins"]
-	path = src/libcompiler_builtins
-	url = https://github.com/rust-lang-nursery/compiler-builtins.git
-[submodule "src/tools/clippy"]
-	path = src/tools/clippy
-	url = https://github.com/rust-lang-nursery/rust-clippy.git
-[submodule "src/tools/rustfmt"]
-	path = src/tools/rustfmt
-	url = https://github.com/rust-lang-nursery/rustfmt.git
-[submodule "src/tools/miri"]
-	path = src/tools/miri
-	url = https://github.com/solson/miri.git
 [submodule "src/dlmalloc"]
 	path = src/dlmalloc
 	url = https://github.com/alexcrichton/dlmalloc-rs.git
-[submodule "src/doc/rust-by-example"]
-	path = src/doc/rust-by-example
-	url = https://github.com/rust-lang/rust-by-example.git
 [submodule "src/llvm-emscripten"]
 	path = src/llvm-emscripten
 	url = https://github.com/rust-lang/llvm.git
@@ -56,12 +22,3 @@
 [submodule "src/libbacktrace"]
 	path = src/libbacktrace
 	url = https://github.com/rust-lang-nursery/libbacktrace.git
-[submodule "src/tools/lldb"]
-	path = src/tools/lldb
-	url = https://github.com/rust-lang-nursery/lldb.git
-	branch = rust-release-80-v1
-[submodule "src/tools/clang"]
-	path = src/tools/clang
-	url = https://github.com/rust-lang-nursery/clang.git
-	branch = rust-release-80-v1
-