Update LLVM submodule
This merges upstream `release/13.x` changes to our fork. In particular,
this includes the bugfix https://reviews.llvm.org/
D108608 (see also
https://bugs.llvm.org/show_bug.cgi?id=51637).
The motivation for this is fixing Rust coverage in Fuchsia: https://fxbug.dev/77427.
-Subproject commit ec995b75d71c5f3b9a82fe084259477eb22f74da
+Subproject commit 86c7daa627aefda1371568b948eba21edf720747