[circt-verilog-lsp][NFC] Break out implementation classes

**Summary**
- Refactored `VerilogServerImpl` into smaller, well-structured components for clarity, testability, and maintainability.
- Moved large inline class implementations out of `VerilogServer.cpp` into dedicated compilation units.

**Changes**
- **New modules**
  - `VerilogTextFile`: manages LSP file contents, versioning, and incremental updates.
  - `VerilogDocument`: handles Slang integration, parsing, diagnostics, and indexing setup.
  - `VerilogIndex`: implements symbol/reference tracking using interval maps.
  - `LSPDiagnosticClient`: converts Slang diagnostics to LSP diagnostics.
  - `VerilogServerContext`: stores server-wide options and shared configuration.
- **Refactors**
  - `VerilogServer`: simplified to a coordinator using the pImpl pattern and `VerilogTextFile` instances.
  - Removed large class definitions (Document, Index, DiagnosticClient, etc.) from `VerilogServer.cpp`.
  - Updated `CMakeLists.txt` to compile new files.
- **Headers**
  - Added clear LLVM-style header comments.
  - Reduced includes and replaced them with forward declarations where possible.

**Impact**
- No functional changes; purely structural refactor.
- Improved code organization and readability.
- Enables independent testing and clearer layering between LSP, Slang, and CIRCT logic.

Author: Moritz Scherer

lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/CMakeLists.txt

@@ -3,6 +3,10 @@ include(SlangCompilerOptions)
 
 add_circt_library(CIRCTVerilogLspServerImpl
   VerilogServer.cpp
+  VerilogTextFile.cpp
+  VerilogDocument.cpp
+  VerilogIndex.cpp
+  LSPDiagnosticClient.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/circt/Tools/circt-verilog-lsp-server

lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/LSPDiagnosticClient.cpp

@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+/// A converter that can be plugged into a slang `DiagnosticEngine` as a
+/// client that will map slang diagnostics to LSP diagnostics.
+
+#include "LSPDiagnosticClient.h"
+
+using namespace circt::lsp;
+
+static llvm::lsp::DiagnosticSeverity
+getSeverity(slang::DiagnosticSeverity severity) {
+  switch (severity) {
+  case slang::DiagnosticSeverity::Fatal:
+  case slang::DiagnosticSeverity::Error:
+    return llvm::lsp::DiagnosticSeverity::Error;
+  case slang::DiagnosticSeverity::Warning:
+    return llvm::lsp::DiagnosticSeverity::Warning;
+  case slang::DiagnosticSeverity::Ignored:
+  case slang::DiagnosticSeverity::Note:
+    return llvm::lsp::DiagnosticSeverity::Information;
+  }
+  llvm_unreachable("all slang diagnostic severities should be handled");
+  return llvm::lsp::DiagnosticSeverity::Error;
+}
+
+void LSPDiagnosticClient::report(const slang::ReportedDiagnostic &slangDiag) {
+  auto loc = document.getLspLocation(slangDiag.location);
+  // Show only the diagnostics in the current file.
+  if (loc.uri != document.getURI())
+    return;
+  auto &mlirDiag = diags.emplace_back();
+  mlirDiag.severity = getSeverity(slangDiag.severity);
+  mlirDiag.range = loc.range;
+  mlirDiag.source = "slang";
+  mlirDiag.message = slangDiag.formattedMessage;
+}

lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/LSPDiagnosticClient.h

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+#include "VerilogDocument.h"
+
+namespace circt {
+namespace lsp {
+
+class LSPDiagnosticClient : public slang::DiagnosticClient {
+  const VerilogDocument &document;
+  std::vector<llvm::lsp::Diagnostic> &diags;
+
+public:
+  LSPDiagnosticClient(const VerilogDocument &document,
+                      std::vector<llvm::lsp::Diagnostic> &diags)
+      : document(document), diags(diags) {}
+
+  void report(const slang::ReportedDiagnostic &slangDiag) override;
+};
+} // namespace lsp
+} // namespace circt