"Function builders" are being renamed to "result builders". Add the
corresponding `@resultBuilder` attribute, with `@_functionBuilder` as
an alias for it, Update test cases to use @resultBuilder.
`PreCheckFunctionBuilderRequest` applies `PreCheckExpression` to the
expressions inside the function body. Previously it used to receive only
`AnyFunctionRef` (`FunctionDecl` or `ClosureExpr`) as the parameter.
However, when fast-completion kicks-in, it replaces the body of the
function, then tries to call `PreCheckFunctionBuilderRequest` again, with
the same function decl as before. It used to return cached "Success"
result, but it didn't actually apply `PreCheckExpression`. So any
`UnresolvedDeclRefExpr` remained unresolved.
In this patch, make `PreCheckFunctionBuilderRequest` receive "body" of the
function as well, so it doesn't return the cached result for the *previous*
body.
rdar://problem/65692922
