[ClangImporter] Handle lookup tables with many entries (#17265)

In particular, there are frameworks with more than 65536 entries for
'init' as a base name.

I can't think of a test case for this besides actually adding such a
massive framework, but the assertions that were added here should
catch the issue in Debug builds if it ever comes up again. I'm going
to go harden everything else that uses this API next.

rdar://problem/40828315

Author: jrose-apple

lib/ClangImporter/SwiftLookupTable.cpp

@@ -961,6 +961,7 @@ namespace {
       if (key.Kind == DeclBaseName::Kind::Normal) {
         keyLength += key.Name.size(); // The name's length
       }
+      assert(keyLength == static_cast<uint16_t>(keyLength));
 
       // # of entries
       uint32_t dataLength = sizeof(uint16_t);
@@ -982,7 +983,7 @@ namespace {
 
       endian::Writer<little> writer(out);
       writer.write<uint16_t>(keyLength);
-      writer.write<uint16_t>(dataLength);
+      writer.write<uint32_t>(dataLength);
       return { keyLength, dataLength };
     }
 
@@ -1067,10 +1068,12 @@ namespace {
       uint32_t keyLength = 1;
       if (SwiftLookupTable::contextRequiresName(key.first))
         keyLength += key.second.size();
+      assert(keyLength == static_cast<uint16_t>(keyLength));
 
       // # of entries
       uint32_t dataLength =
         sizeof(uint16_t) + sizeof(uint64_t) * data.size();
+      assert(dataLength == static_cast<uint16_t>(dataLength));
 
       endian::Writer<little> writer(out);
       writer.write<uint16_t>(keyLength);
@@ -1226,7 +1229,7 @@ namespace {
     static std::pair<unsigned, unsigned>
     ReadKeyDataLength(const uint8_t *&data) {
       unsigned keyLength = endian::readNext<uint16_t, little, unaligned>(data);
-      unsigned dataLength = endian::readNext<uint16_t, little, unaligned>(data);
+      unsigned dataLength = endian::readNext<uint32_t, little, unaligned>(data);
       return { keyLength, dataLength };
     }