[libc] Template the printf / scanf parser class (#66277)

Summary:
The parser class for stdio currently accepts different argument
providers. In-tree this is only used for a fuzzer test, however, the
proposed implementation of the GPU handling of printf / scanf will
require custom argument handlers. This makes the current approach of
using a preprocessor macro messier. This path proposed folding this
logic into a template instantiation. The downside to this is that
because the implementation of the parser class is placed into an
implementation file we need to manually instantiate the needed templates
which will slightly bloat binary size. Alternatively we could remove the
implementation file, or key off of the `libc` external packaging macro
so it is not present in the installed version.

libc/fuzzing/stdio/CMakeLists.txt

@@ -3,9 +3,7 @@ add_libc_fuzzer(
   SRCS
     printf_parser_fuzz.cpp
   DEPENDS
-    libc.src.stdio.printf_core.mock_parser
-  COMPILE_OPTIONS
-    -DLIBC_COPT_MOCK_ARG_LIST
+    libc.src.stdio.printf_core.parser
 )
 
 add_libc_fuzzer(

libc/fuzzing/stdio/printf_parser_fuzz.cpp

@@ -10,10 +10,6 @@
 ///
 //===----------------------------------------------------------------------===//
 
-#ifndef LIBC_COPT_MOCK_ARG_LIST
-#error The printf Parser Fuzzer must be compiled with LIBC_COPT_MOCK_ARG_LIST, and the parser itself must also be compiled with that option when it's linked against the fuzzer.
-#endif
-
 #include "src/__support/arg_list.h"
 #include "src/stdio/printf_core/parser.h"
 
@@ -37,7 +33,8 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 
   auto mock_arg_list = internal::MockArgList();
 
-  auto parser = printf_core::Parser(in_str, mock_arg_list);
+  auto parser =
+      printf_core::Parser<internal::MockArgList>(in_str, mock_arg_list);
 
   int str_percent_count = 0;