[clangd] Don't use zlib when it's unavailable.

Without this patch `clangd` crashes at try to load compressed string table when `zlib` is not available.
Example:
- Build `clangd` with MinGW (`zlib` found)
- Build index
- Build `clangd` with Visual Studio compiler (`zlib` not found)
- Try to load index

Reviewed By: sammccall, adamcz

Differential Revision: https://reviews.llvm.org/D87673

clang-tools-extra/clangd/index/Serialization.cpp

@@ -201,12 +201,13 @@ llvm::Expected<StringTableIn> readStringTable(llvm::StringRef Data) {
   llvm::SmallString<1> UncompressedStorage;
   if (UncompressedSize == 0) // No compression
     Uncompressed = R.rest();
-  else {
+  else if (llvm::zlib::isAvailable()) {
     if (llvm::Error E = llvm::zlib::uncompress(R.rest(), UncompressedStorage,
                                                UncompressedSize))
       return std::move(E);
     Uncompressed = UncompressedStorage;
-  }
+  } else
+    return error("Compressed string table, but zlib is unavailable");
 
   StringTableIn Table;
   llvm::StringSaver Saver(Table.Arena);