llgo: Fix use of llvm.trap
llgo: Update llgo to latest go/types

All tests pass, except the method expression/selector one,
which has been temporarily disabled. This will be resolved
in a future commit.
llgo: fix method expressions

All unit tests passing now.
llgo: fix regression on imported funcs/vars
Various fixes

llgo can once again build programs that use pkg/fmt.
Fix creation of global string constants

Don't use Builder in NewConstValue.