feat(compiler): Add class StreamStringError with a stream interface for llvm::Error

Composing error messages for `llvm::Error` is either done by using
`llvm::createStringError()` with an appropriate format string and
arguments or by writing to a `std::string`-backed
`llvm::raw_string_ostream` and passing the result to
`llvm::make_error<llvm::StringError>()` verbatim.

The new class `StreamStringError` encapsulates the latter solution
into a class with an appropriate stream operator and implicit cast
operators to `llvm::Error` and `llvm::Expected`.

Example usage:

   llvm::Error foo(int i, size_t s, ...) {
      ...
      if(...) {
        return StreamStringError()
               << "Some error message with an integer: "
               << i << " and a size_t: " << s;
      }
      ...
   }

compiler/include/zamalang/Support/Error.h

@@ -0,0 +1,53 @@
+#ifndef ZAMALANG_SUPPORT_STRING_ERROR_H
+#define ZAMALANG_SUPPORT_STRING_ERROR_H
+
+#include <llvm/Support/Error.h>
+
+namespace mlir {
+namespace zamalang {
+
+// Internal error class that allows for composing `llvm::Error`s
+// similar to `llvm::createStringError()`, but using stream-like
+// composition with `operator<<`.
+//
+// Example:
+//
+//   llvm::Error foo(int i, size_t s, ...) {
+//      ...
+//      if(...) {
+//        return StreamStringError()
+//               << "Some error message with an integer: "
+//               << i << " and a size_t: " << s;
+//      }
+//      ...
+//   }
+class StreamStringError {
+public:
+  StreamStringError(const llvm::StringRef &s) : buffer(s.str()), os(buffer){};
+  StreamStringError() : buffer(""), os(buffer){};
+
+  template <typename T> StreamStringError &operator<<(const T &v) {
+    this->os << v;
+    return *this;
+  }
+
+  operator llvm::Error() {
+    return llvm::make_error<llvm::StringError>(os.str(),
+                                               llvm::inconvertibleErrorCode());
+  }
+
+  template <typename T> operator llvm::Expected<T>() {
+    return this->operator llvm::Error();
+  }
+
+protected:
+  std::string buffer;
+  llvm::raw_string_ostream os;
+};
+
+StreamStringError &operator<<(StreamStringError &se, llvm::Error &err);
+
+} // namespace zamalang
+} // namespace mlir
+
+#endif