all: add LLVM 12 support

Originally based on a PR by @QuLogic, but extended a lot to get all
tests to pass.

cgo/libclang_config.go

@@ -1,4 +1,5 @@
 // +build !byollvm
+// +build !llvm12
 
 package cgo
 

cgo/libclang_config_llvm12.go

@@ -0,0 +1,14 @@
+// +build !byollvm
+// +build llvm12
+
+package cgo
+
+/*
+#cgo linux   CFLAGS:  -I/usr/lib/llvm-12/include
+#cgo darwin  CFLAGS:  -I/usr/local/opt/llvm@12/include
+#cgo freebsd CFLAGS:  -I/usr/local/llvm12/include
+#cgo linux   LDFLAGS: -L/usr/lib/llvm-12/lib -lclang
+#cgo darwin  LDFLAGS: -L/usr/local/opt/llvm@12/lib -lclang -lffi
+#cgo freebsd LDFLAGS: -L/usr/local/llvm12/lib -lclang
+*/
+import "C"