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.

.github/workflows/sizediff.yml

@@ -24,19 +24,19 @@ jobs:
           submodules: true
       - name: Install apt dependencies
         run: |
-          echo 'deb https://apt.llvm.org/jammy/ llvm-toolchain-jammy-15 main' | sudo tee /etc/apt/sources.list.d/llvm.list
+          echo 'deb https://apt.llvm.org/jammy/ llvm-toolchain-jammy-16 main' | sudo tee /etc/apt/sources.list.d/llvm.list
           wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
           sudo apt-get update
           sudo apt-get install --no-install-recommends -y \
-              llvm-15-dev \
-              clang-15 \
-              libclang-15-dev \
-              lld-15
+              llvm-16-dev \
+              clang-16 \
+              libclang-16-dev \
+              lld-16
       - name: Restore LLVM source cache
         uses: actions/cache@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-15-sizediff-v1
+          key: llvm-source-16-sizediff-v1
           path: |
             llvm-project/compiler-rt
       - name: Download LLVM source