Switch benchmarks to match unit test namespacing. (#3268)

Specifically, rather than nesting them in `Carbon::Testing`, nest them
in `Carbon::Foo` for whatever component they're benchmarking. All our
current benchmarks are lexer benchmarks so its `Carbon::Lex`.

This makes even more sense for benchmarks than unittests I think.

toolchain/lex/string_literal_benchmark.cpp

@@ -7,11 +7,9 @@
 #include "toolchain/diagnostics/null_diagnostics.h"
 #include "toolchain/lex/string_literal.h"
 
-namespace Carbon::Testing {
+namespace Carbon::Lex {
 namespace {
 
-using Lex::StringLiteral;
-
 static void BM_ValidString(benchmark::State& state, std::string_view introducer,
                            std::string_view terminator) {
   std::string x(introducer);
@@ -117,4 +115,4 @@ BENCHMARK(BM_SimpleStringValue_MultilineDoubleQuote);
 BENCHMARK(BM_SimpleStringValue_Raw);
 
 }  // namespace
-}  // namespace Carbon::Testing
+}  // namespace Carbon::Lex