all: switch to LLVM 16

This commit adds support for LLVM 16 and switches to it by default. That
means three LLVM versions are supported at the same time: LLVM 14, 15,
and 16.

This commit includes work by QuLogic:

  * Part of this work was based on a PR by QuLogic:
    https://github.com/tinygo-org/tinygo/pull/3649
    But I also had parts of this already implemented in an old branch I
    already made for LLVM 16.
  * QuLogic also provided a CGo fix here, which is also incorporated in
    this commit:
    https://github.com/tinygo-org/tinygo/pull/3869

The difference with the original PR by QuLogic is that this commit is
more complete:
  * It switches to LLVM 16 by default.
  * It updates some things to also make it work with a self-built LLVM.
  * It fixes the CGo bug in a slightly different way, and also fixes
    another one not included in the original PR.
  * It does not keep compiler tests passing on older LLVM versions. I
    have found this to be quite burdensome and therefore don't generally
    do this - the smoke tests should hopefully catch most regressions.

cgo/libclang_config_llvm16.go

@@ -0,0 +1,21 @@
+//go:build !byollvm && !llvm14 && !llvm15
+
+package cgo
+
+// As of 2023-05-05, there is a packaging issue on Debian:
+// https://github.com/llvm/llvm-project/issues/62199
+// A workaround is to fix this locally, using something like this:
+//
+//   ln -sf ../../x86_64-linux-gnu/libclang-16.so.1 /usr/lib/llvm-16/lib/libclang.so
+
+/*
+#cgo linux        CFLAGS:  -I/usr/lib/llvm-16/include
+#cgo darwin,amd64 CFLAGS:  -I/usr/local/opt/llvm@16/include
+#cgo darwin,arm64 CFLAGS:  -I/opt/homebrew/opt/llvm@16/include
+#cgo freebsd      CFLAGS:  -I/usr/local/llvm16/include
+#cgo linux        LDFLAGS: -L/usr/lib/llvm-16/lib -lclang
+#cgo darwin,amd64 LDFLAGS: -L/usr/local/opt/llvm@16/lib -lclang -lffi
+#cgo darwin,arm64 LDFLAGS: -L/opt/homebrew/opt/llvm@16/lib -lclang -lffi
+#cgo freebsd      LDFLAGS: -L/usr/local/llvm16/lib -lclang
+*/
+import "C"