[cfe] Enum named _ issue

[sgrekhov]

The test https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Wildcards/other_declarations_A04_t07.dart fails on CFE (works well in the analyzer).

/=======================================================================================================================\
| co19/LanguageFeatures/Wildcards/other_declarations_A04_t07 is new and failed (MissingCompileTimeError, expected Pass) |
\=======================================================================================================================/

--- Command "fasta" (took 21ms):
(cd /b/s/w/ir/cache/builder/sdk/ ; DART_CONFIGURATION=ReleaseX64 ./out/ReleaseX64/dart ./pkg/front_end/tool/_fasta/compile.dart --verify --skip-platform-verification -o ./out/ReleaseX64/generated_compilations/cfe-strong-linux/tests_co19_src_LanguageFeatures_Wildcards_other_declarations_A04_t07/out.dill --platform ./out/ReleaseX64/vm_platform_strong.dill -Dtest_runner.configuration=cfe-strong-linux --enable-experiment=wildcard-variables --packages=./.dart_tool/package_config.json ./tests/co19/src/LanguageFeatures/Wildcards/other_declarations_A04_t07.dart )

static error failures:
- Unexpected error at tests/co19/src/LanguageFeatures/Wildcards/other_declarations_A04_t07.dart line 15, column 9: Couldn't find constructor '_'.

--- Re-run this test:
python3 tools/test.py -n cfe-strong-linux co19/LanguageFeatures/Wildcards/other_declarations_A04_t07

I don't see anything wrong on line 15

// SharedOptions=--enable-experiment=wildcard-variables

enum _ {e1;} // Line 15

int _ = 1;
//  ^
// [analyzer] unspecified
// [cfe] unspecified

const _ = 1;
//    ^
// [analyzer] unspecified
// [cfe] unspecified
...

The following code produces no issues in both CFE and analyzer

enum _ {e1;}

main() {
  print(_);
}