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/windows.yml

@@ -41,7 +41,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-15-windows-v4
+          key: llvm-source-16-windows-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -66,7 +66,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-windows-v6
+          key: llvm-build-16-windows-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'