[NFC] Fix some C++20 warnings

[Godin]

Summary

Without this patch when using CMAKE_CXX_STANDARD=20 Microsoft compiler produces following warnings

clang\include\clang/Basic/DiagnosticIDs.h(48): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(49): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(50): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(51): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(52): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(53): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(54): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(55): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(56): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(57): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(58): warning C5054: operator '+': deprecated between enumerations of different types
clang\include\clang/Basic/DiagnosticIDs.h(59): warning C5054: operator '+': deprecated between enumerations of different types

[aaron.ballman]

LGTM, thank you!

[Godin]

@aaron.ballman thanks for the review! could you please commit this on my behalf since I do not have commit rights?

clang/include/clang/Basic/DiagnosticIDs.h

 //===--- DiagnosticIDs.h - Diagnostic IDs Handling --------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 ///
enum {
       DIAG_SIZE_CROSSTU       =  100,
       DIAG_SIZE_SEMA          = 4500,
       DIAG_SIZE_ANALYSIS      =  100,
       DIAG_SIZE_REFACTORING   = 1000,
     };
     // Start position for diagnostics.
     enum {
       DIAG_START_COMMON        =                          0,
-      DIAG_START_DRIVER        = DIAG_START_COMMON        + DIAG_SIZE_COMMON,
-      DIAG_START_FRONTEND      = DIAG_START_DRIVER        + DIAG_SIZE_DRIVER,
-      DIAG_START_SERIALIZATION = DIAG_START_FRONTEND      + DIAG_SIZE_FRONTEND,
-      DIAG_START_LEX           = DIAG_START_SERIALIZATION + DIAG_SIZE_SERIALIZATION,
-      DIAG_START_PARSE         = DIAG_START_LEX           + DIAG_SIZE_LEX,
-      DIAG_START_AST           = DIAG_START_PARSE         + DIAG_SIZE_PARSE,
-      DIAG_START_COMMENT       = DIAG_START_AST           + DIAG_SIZE_AST,
-      DIAG_START_CROSSTU       = DIAG_START_COMMENT       + DIAG_SIZE_COMMENT,
-      DIAG_START_SEMA          = DIAG_START_CROSSTU       + DIAG_SIZE_CROSSTU,
-      DIAG_START_ANALYSIS      = DIAG_START_SEMA          + DIAG_SIZE_SEMA,
-      DIAG_START_REFACTORING   = DIAG_START_ANALYSIS      + DIAG_SIZE_ANALYSIS,
-      DIAG_UPPER_LIMIT         = DIAG_START_REFACTORING   + DIAG_SIZE_REFACTORING
+      DIAG_START_DRIVER        = DIAG_START_COMMON        + static_cast<int>(DIAG_SIZE_COMMON),
+      DIAG_START_FRONTEND      = DIAG_START_DRIVER        + static_cast<int>(DIAG_SIZE_DRIVER),
+      DIAG_START_SERIALIZATION = DIAG_START_FRONTEND      + static_cast<int>(DIAG_SIZE_FRONTEND),
+      DIAG_START_LEX           = DIAG_START_SERIALIZATION + static_cast<int>(DIAG_SIZE_SERIALIZATION),
+      DIAG_START_PARSE         = DIAG_START_LEX           + static_cast<int>(DIAG_SIZE_LEX),
+      DIAG_START_AST           = DIAG_START_PARSE         + static_cast<int>(DIAG_SIZE_PARSE),
+      DIAG_START_COMMENT       = DIAG_START_AST           + static_cast<int>(DIAG_SIZE_AST),
+      DIAG_START_CROSSTU       = DIAG_START_COMMENT       + static_cast<int>(DIAG_SIZE_COMMENT),
+      DIAG_START_SEMA          = DIAG_START_CROSSTU       + static_cast<int>(DIAG_SIZE_CROSSTU),
+      DIAG_START_ANALYSIS      = DIAG_START_SEMA          + static_cast<int>(DIAG_SIZE_SEMA),
+      DIAG_START_REFACTORING   = DIAG_START_ANALYSIS      + static_cast<int>(DIAG_SIZE_ANALYSIS),
+      DIAG_UPPER_LIMIT         = DIAG_START_REFACTORING   + static_cast<int>(DIAG_SIZE_REFACTORING)
     };
 
     class CustomDiagInfo;
 
     /// All of the diagnostics that can be emitted by the frontend.
     typedef unsigned kind;
 
     // Get typedefs for common diagnostics.