…tax enabled

…ly mode

…swift-syntax

When building without swift-syntax, we don't have the ability to handle
macros. However, the concurrency runtime makes use of `#isolation` in
a number of places. To continue supporting building without
swift-syntax, parse `#isolation` directly into the otherwise-unspellable
CurrentContextIsolationExpr but *only* when swift-syntax is not available.

Based on a patch from Evan Wilde, introduce a new preset that disables
the features that require Swift host tools. This involves disabling
early swift-syntax (used in the compiler for macros et al) and early
swift-driver.

These annotations require optimization passes that are written in
Swift, so we need a host Swift compiler to properly process them. Use
`hasFeature(Macros)` as a way to gate the annotations.

… written in Swift