Change StringLiteral to less frequently allocate a new string. (#3314)

Building on #3311, which started moving the result string into a
`unique_ptr`, instead have `StringLiteral` use a `BumpPtrAllocator` to
manage memory. But also, detect when a string is really trivial during
`Lex` and, if so, return `contents_` directly.

toolchain/lex/string_literal_fuzzer.cpp

@@ -33,8 +33,9 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
   // Check multiline flag was computed correctly.
   CARBON_CHECK(token->is_multi_line() == token->text().contains('\n'));
 
+  llvm::BumpPtrAllocator allocator;
   volatile auto value =
-      token->ComputeValue(NullDiagnosticEmitter<const char*>());
+      token->ComputeValue(allocator, NullDiagnosticEmitter<const char*>());
   (void)value;
 
   return 0;