Most of AST, Parse, and Sema deal with FileUnits regularly, but SIL
and IRGen certainly don't. Split FileUnit out into its own header to
cut down on recompilation times when something changes.
No functionality change.
The build on Windows has been failing due to the ambiguous constructor
overload:
```
lib\Parse\ParseRequests.cpp(83): error C2666: 'swift::Parser::Parser': 2 overloads have similar conversions
include\swift\Parse\Parser.h(412): note: could be 'swift::Parser::Parser(std::unique_ptr<swift::Lexer,std::default_delete<_Ty>>,swift::SourceFile &,swift::SILParserTUStateBase *,swift::PersistentParserState *,std::shared_ptr<swift::SyntaxParseActions>,bool)'
with
[
_Ty=swift::Lexer
]
include\swift\Parse\Parser.h(408): note: or 'swift::Parser::Parser(unsigned int,swift::SourceFile &,swift::SILParserTUStateBase *,swift::PersistentParserState *,std::shared_ptr<swift::SyntaxParseActions>,bool)'
include\swift\Parse\Parser.h(403): note: or 'swift::Parser::Parser(unsigned int,swift::SourceFile &,swift::DiagnosticEngine *,swift::SILParserTUStateBase *,swift::PersistentParserState *,std::shared_ptr<swift::SyntaxParseActions>,bool)'
lib\Parse\ParseRequests.cpp(84): note: while trying to match the argument list '(unsigned int, swift::SourceFile, nullptr, nullptr, nullptr, bool)'
```
Rework the lazy function body parsing mechanism to use the
request-evaluator, so that asking for the body of a function will
initiate parsing. Clean up a number of callers to
AbstractFunctionDecl::getBody() that don't actually need the body, so
we don't perform unnecessary parsing.
This change does not delay parsing of function bodies in the general
case; rather, it sets up the infrastructure to always delay parsing of
function bodies.
Rework the lazy function body parsing mechanism to use the
request-evaluator, so that asking for the body of a function will
initiate parsing. Clean up a number of callers to
AbstractFunctionDecl::getBody() that don't actually need the body, so
we don't perform unnecessary parsing.
This change does not delay parsing of function bodies in the general
case; rather, it sets up the infrastructure to always delay parsing of
function bodies.
Lazy parsing for the members of nominal types and extensions depends
only on information already present in
`IterableDeclContext`. Eliminate the use of PersistentParserState as
an intermediary and have the member-parsing request construct a new
`Parser` instance itself to handle parsing. Make this possible even
for ill-formed nominal types/extensions to simplify the code path.
Eliminate `LazyMemberParser` and all of its uses, because it was only
present for lazy member parsing, which no longer needs it.
Ensure that lazy parsing of the members of nominal type definitions
and extensions is handled through a request. Most of the effort here
is in establishing a new request zone for parser requests.
