all: remove LLVM 14 support

This is a big change: apart from removing LLVM 14 it also removes typed
pointer support (which was only fully supported in LLVM up to version
14). This removes about 200 lines of code, but more importantly removes
a ton of special cases for LLVM 14.

.circleci/config.yml

@@ -98,12 +98,12 @@ commands:
             - /go/pkg/mod
 
 jobs:
-  test-llvm14-go118:
+  test-llvm15-go118:
     docker:
       - image: golang:1.18-buster
     steps:
       - test-linux:
-          llvm: "14"
+          llvm: "15"
     resource_class: large
 
 workflows:
@@ -111,4 +111,4 @@ workflows:
     jobs:
       # This tests our lowest supported versions of Go and LLVM, to make sure at
       # least the smoke tests still pass.
-      - test-llvm14-go118
+      - test-llvm15-go118