Auto merge of rust-lang#125288 - nikic:update-llvm-18.1.6, r=cuviper

Update to LLVM 18.1.6

This rebases our LLVM fork on top of LLVM 18.1.6, which is planned to be the last release of the 18.x series.

Fixes rust-lang#123695.
Fixes rust-lang#125053.

r? `@cuviper`

Author: bors

.gitmodules

@@ -33,7 +33,7 @@
 [submodule "src/llvm-project"]
 	path = src/llvm-project
 	url = https://github.com/rust-lang/llvm-project.git
-	branch = rustc/18.0-2024-02-13
+	branch = rustc/18.1-2024-05-19
 	shallow = true
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book