[clangd] Use enumMember instead of enumConstant

We should be using enumMember as thats defined in LSP, enumConstant is non standard so clients aren't likely to support it
Fixes https://github.com/clangd/clangd/issues/622n

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D93113

Author: njames93

clang-tools-extra/clangd/SemanticHighlighting.cpp

@@ -571,7 +571,7 @@ llvm::StringRef toSemanticTokenType(HighlightingKind Kind) {
   case HighlightingKind::Enum:
     return "enum";
   case HighlightingKind::EnumConstant:
-    return "enumConstant"; // nonstandard
+    return "enumMember";
   case HighlightingKind::Typedef:
     return "type";
   case HighlightingKind::DependentType: