Use llvm:: for specializations instead of 'namespace llvm' (#3682)

We already do this with things like llvm::DenseMapInfo, I don't know why
I was doing this with format_provider. But this should be more
consistent, and slightly better for not entering another library's
namespace.

toolchain/parse/context.cpp

@@ -25,11 +25,9 @@ enum class RelativeLocation : int8_t {
 
 }  // namespace Carbon::Parse
 
-namespace llvm {
-
 // Adapts RelativeLocation for use with formatv.
 template <>
-struct format_provider<Carbon::Parse::RelativeLocation> {
+struct llvm::format_provider<Carbon::Parse::RelativeLocation> {
   using RelativeLocation = Carbon::Parse::RelativeLocation;
   static void format(const RelativeLocation& loc, raw_ostream& out,
                      StringRef /*style*/) {
@@ -47,8 +45,6 @@ struct format_provider<Carbon::Parse::RelativeLocation> {
   }
 };
 
-}  // namespace llvm
-
 namespace Carbon::Parse {
 
 Context::Context(Tree& tree, Lex::TokenizedBuffer& tokens,