[clang-format] CSharp don't allow there not to be a space between `is` and `[`

[MyDeveloperDay]

Summary

as is is a keyword in C# ensure there is always a space before the [ regardless of SpaceBeforeSquareBrackets setting

Fixes: #61965

https://github.com/llvm/llvm-project/issues/61965

[owenpan]

clang/unittests/Format/FormatTestCSharp.cpp

1182–1186

Should we end the return statements with a semicolon?

clang/lib/Format/TokenAnnotator.cpp

if (Previous) {
       if (Previous->is(tok::kw_const) || Previous->is(tok::kw_volatile)) {
         return (Style.PointerAlignment != FormatStyle::PAS_Left) ||
                (Style.SpaceAroundPointerQualifiers ==
                 FormatStyle::SAPQ_After) ||
                (Style.SpaceAroundPointerQualifiers == FormatStyle::SAPQ_Both);
       }
     }
   }
+  if (Style.isCSharp() && Left.is(Keywords.kw_is) && Right.is(tok::l_square))
+    return true;
   const auto SpaceRequiredForArrayInitializerLSquare =
       [](const FormatToken &LSquareTok, const FormatStyle &Style) {
         return Style.SpacesInContainerLiterals ||
                ((Style.Language == FormatStyle::LK_Proto ||
                  Style.Language == FormatStyle::LK_TextProto) &&
                 !Style.Cpp11BracedListStyle &&
                 LSquareTok.endsSequence(tok::l_square, tok::colon,
                                         TT_SelectorName));

clang/unittests/Format/FormatTestCSharp.cpp

foreach ((A a, B b) in someList) {
   verifyFormat(R"(extern (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(static (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(internal (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(abstract (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(sealed (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(override (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(async (string name, int age) methodTuple() {})", Style);
   verifyFormat(R"(unsafe (string name, int age) methodTuple() {})", Style);
+
+  Style.SpacesInSquareBrackets = false;
+  Style.SpaceBeforeSquareBrackets = true;
+  verifyFormat("return a is [1, 2, 3]", Style);
+  verifyFormat("return a is [..]", Style);
+  Style.SpaceBeforeSquareBrackets = false;
+  verifyFormat("return a is [1, 2, 3]", Style);
+  verifyFormat("return a is [..]", Style);
 }
 
 TEST_F(FormatTestCSharp, CSharpNullableTypes) {
   FormatStyle Style = getGoogleStyle(FormatStyle::LK_CSharp);
   Style.SpacesInSquareBrackets = false;
 
   verifyFormat(R"(//
 public class A {