Researching in prep of doing the implementation for lowering, I found
that the source of the valid identifiers list from flang is in the
frontend. This patch adds the same list to the frontend, but does it as
a sema diagnostic, so we still parse it as an identifier/identifier-like
thing, but then diagnose it as invalid later.
This is a very simple sema implementation, and just required AST node
plus the existing diagnostics. This patch adds tests and adds the AST
node required, plus enables it for 'init' and 'shutdown' (only!)
These two constructs are very simple and similar, and only support 3
different clauses, two of which are already implemented. This patch
adds AST nodes for both constructs, and leaves the device_num clause
unimplemented, but enables the other two.
