An early approach to codegen for `#_hasSymbol` relied on the Darwin platfom SDK, but now that the feature lowers directly to NULL checks in LLVM IR a platform restriction is no longer needed.
However, the tests for `#_hasSymbol` remain unsupported on Windows since that OS does not support weak linking.
Type checking of closure bodies does not invoke `TypeChecker::typeCheckStmtConditionElement()` so use of availability macros inside closure bodies written in fragile (inlinable) functions was undiagnosed. Move the diagnostic logic to `MiscDiagnostics.cpp` which implements diagnostics that are emitted regardless of the typechecking model used.
Additionally, remove the `DC->getAsDecl() != nullptr` check from `diagnoseHasSymbolCondition()` which was preventing similar fragile function diagnostics for `if #_hasSymbol` from being emitted when a closure was involved.
Resolves rdar://100581013
- Unapplied functions: `if #_hasSymbol(foo(_:))`
- Member references: `if #_hasSymbol(a.x)`
- `.self` expressions on static metatypes: `if #_hasSymbol(SomeEnum.self)`
- Dot syntax call expressions with unapplied functions: `if #_hasSymbol(SomeType.init(_:)`
Additionally, we diagnose when the concretely referenced declaration is not weak linked since this likely indicates that the programmer misidentified the declaration they wish to check or the build is not configured in a way such that the check will be meaningful.
Resolves rdar://99826340
