Add Lexer::IsHashbangAllowed, drop SourceManager::getHashbangBufferID (#18534)

Having this be a single buffer hardcoded in the SourceManager and set
by all clients is silly. SourceFiles with the 'Main' kind are allowed
to have hashbang lines (`#!`), other files are not. And anyone
manually setting up a Lexer can decide for themselves.

No intended behavioral change.

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -506,13 +506,7 @@ static StringRef getSourceToken(unsigned Offset,
                                                  MemBuf->getBufferIdentifier());
   auto BufId = SM.addNewSourceBuffer(std::move(MemBufRef));
   SourceLoc Loc = SM.getLocForOffset(BufId, Offset);
-
-  // Use fake language options; language options only affect validity
-  // and the exact token produced.
-  LangOptions FakeLangOpts;
-  Lexer L(FakeLangOpts, SM, BufId, nullptr, /*InSILMode=*/ false,
-          CommentRetentionMode::ReturnAsTokens);
-  return L.getTokenAt(Loc).getText();
+  return Lexer::getTokenAtLocation(SM, Loc).getText();
 }
 
 static llvm::Optional<unsigned>