[Diagnostics] Eliminate silent exits on precondition violations

The diagnostic bridging code from C++ diagnostics to swift-syntax was
silently returning on certain preconditions. These should never
happen, and if they did would silently drop diagnostics on the floor,
leading to much consternation. Turn them into proper fatalErrors.

Author: DougGregor

lib/ASTGen/Sources/ASTGen/DiagnosticsBridge.swift

@@ -267,7 +267,7 @@ public func addQueuedDiagnostic(
   )
 
   guard let rawPosition = cLoc.getOpaquePointerValue() else {
-    return
+    fatalError("Cannot emit diagnostic with no source location here")
   }
 
   // Find the source file that contains this location.
@@ -279,8 +279,7 @@ public func addQueuedDiagnostic(
     return rawPosition >= baseAddress && rawPosition <= baseAddress + sf.buffer.count
   }
   guard let sourceFile = sourceFile else {
-    // FIXME: Hard to report an error here...
-    return
+    fatalError("Missing source file for diagnostic")
   }
 
   let sourceFileBaseAddress = UnsafeRawPointer(sourceFile.buffer.baseAddress!)
@@ -302,8 +301,7 @@ public func addQueuedDiagnostic(
       node = sourceFile.syntax
     }
   } else {
-    // position out of range.
-    return
+    fatalError("Diagnostic position is outside of its source file")
   }
 
   // Map the highlights.