[Parse] Allow skipping member bodies following #sourceLocation

If the bodies themselves don't have a
`#sourceLocation` in them, there should be no
harm in skipping; the parsed virtual files get
recorded on the SourceManager, so will be
accessible when doing delayed parsing. The
`InPoundLineEnvironment` flag will be inaccurate,
but that's only needed for the parsing of
`#sourceLocation` itself.

Author: hamishknight

include/swift/Parse/Parser.h

@@ -156,7 +156,13 @@ class Parser {
   /// Whether this context has an async attribute.
   bool InPatternWithAsyncAttribute = false;
 
+  /// Whether a '#sourceLocation' is currently active.
+  /// NOTE: Do not use this outside of `parseLineDirective`, it doesn't have
+  /// its state restored when parsing delayed bodies (we avoid skipping bodies
+  /// if we see a `#sourceLocation` in them though). Instead, query the
+  /// SourceManager.
   bool InPoundLineEnvironment = false;
+
   bool InPoundIfEnvironment = false;
   /// ASTScopes are not created in inactive clauses and lookups to decls will fail.
   bool InInactiveClauseEnvironment = false;

lib/Parse/ParseDecl.cpp

@@ -7080,10 +7080,6 @@ bool Parser::canDelayMemberDeclParsing(bool &HasOperatorDeclarations,
   // If explicitly disabled, respect the flag.
   if (!isDelayedParsingEnabled())
     return false;
-  // Recovering parser status later for #sourceLocation is not-trivial and
-  // it may not worth it.
-  if (InPoundLineEnvironment)
-    return false;
 
   // Skip until the matching right curly bracket; if we find a pound directive,
   // we can't lazily parse.