[locale] Avoid copy of __atoms when char_type is char

The function num_get<_CharT>::stage2_int_prep makes unnecessary copy of src
into atoms when char_type is char. This can be avoided by creating
a switch on type and just returning __src when char_type is char.

Added the test case to demonstrate performance improvement.
In order to avoid ABI incompatibilities, the changes are guarded
with a macro _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET

Differential Revision: https://reviews.llvm.org/D30268
Reviewed by: EricWF

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305427 91177308-0d34-0410-b5e6-96231b3b80d8

benchmarks/stringstream.bench.cpp

@@ -0,0 +1,38 @@
+#include "benchmark/benchmark_api.h"
+
+#include <sstream>
+double __attribute__((noinline)) istream_numbers();
+
+double istream_numbers() {
+  const char *a[] = {
+    "-6  69 -71  2.4882e-02 -100 101 -2.00005 5000000 -50000000",
+    "-25 71   7 -9.3262e+01 -100 101 -2.00005 5000000 -50000000",
+    "-14 53  46 -6.7026e-02 -100 101 -2.00005 5000000 -50000000"
+  };
+
+  int a1, a2, a3, a4, a5, a6, a7;
+  double f1 = 0.0, f2 = 0.0, q = 0.0;
+  for (int i=0; i < 3; i++) {
+    std::istringstream s(a[i]);
+    s >> a1
+      >> a2
+      >> a3
+      >> f1
+      >> a4
+      >> a5
+      >> f2
+      >> a6
+      >> a7;
+    q += (a1 + a2 + a3 + a4 + a5 + a6 + a7 + f1 + f2)/1000000;
+  }
+  return q;
+}
+
+static void BM_Istream_numbers(benchmark::State &state) {
+  double i = 0;
+  while (state.KeepRunning())
+    benchmark::DoNotOptimize(i += istream_numbers());
+}
+
+BENCHMARK(BM_Istream_numbers)->RangeMultiplier(2)->Range(1024, 4096);
+BENCHMARK_MAIN()