Bump version to 0.5.0, update README

llvmgen: updates for LLVM 15 and later

Bump version to 0.4.0, update README

llvmgen: finish migration to opaque-pointer APIs

llvmgen_symgen: migrate to opaque pointers

Update minimum LLVM version to 14

llvm_expgen: use llvmgen_rtntype_info in gen_routine_call

llvm_symgent: add llvmgen_rtntype_info function

llvm_helper: update TargetRegistry include

LICENSE.TXT: bump copyright year

README.md: update toolchain versions

CMakeLists: use LLVM 12, update version to 0.3.90

FindLLVM.cmake: look for version-suffixed llvm-config program

lib: fixes for static analysis warnings

llvmgen: update for LLVM 12