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/lex/numeric_literal.cpp

@@ -12,12 +12,10 @@
 #include "toolchain/lex/character_set.h"
 #include "toolchain/lex/helpers.h"
 
-namespace llvm {
-
 // We use formatv primarily for diagnostics. In these cases, it's expected that
 // the spelling in source code should be used.
 template <>
-struct format_provider<Carbon::Lex::NumericLiteral::Radix> {
+struct llvm::format_provider<Carbon::Lex::NumericLiteral::Radix> {
   using Radix = Carbon::Lex::NumericLiteral::Radix;
   static void format(const Radix& radix, raw_ostream& out,
                      StringRef /*style*/) {
@@ -35,8 +33,6 @@ struct format_provider<Carbon::Lex::NumericLiteral::Radix> {
   }
 };
 
-}  // namespace llvm
-
 namespace Carbon::Lex {
 
 auto NumericLiteral::Lex(llvm::StringRef source_text)