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/linux.yml

@@ -43,7 +43,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-15-linux-alpine-v3
+          key: llvm-source-16-linux-alpine-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -68,7 +68,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-linux-alpine-v4
+          key: llvm-build-16-linux-alpine-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -194,7 +194,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-15-linux-asserts-v3
+          key: llvm-source-16-linux-asserts-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -219,7 +219,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-linux-asserts-v4
+          key: llvm-build-16-linux-asserts-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -297,7 +297,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-15-linux-v3
+          key: llvm-source-16-linux-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -322,7 +322,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-linux-arm-v4
+          key: llvm-build-16-linux-arm-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -414,7 +414,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-source
         with:
-          key: llvm-source-15-linux-v3
+          key: llvm-source-16-linux-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -439,7 +439,7 @@ jobs:
         uses: actions/cache/restore@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-linux-arm64-v4
+          key: llvm-build-16-linux-arm64-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'