Add __builtion_unreachable to vector::size(), vector::capacity()

This patch makes it clear that vector sizes and capacities are not
negative.  With recent change to ipa-fnsummary this should not affect
inlining and improves codegen of some vector manipulation functions.

I tested clang build.  Looking for throw_bad calls there are only 3
called considerably often (bad_allloc, bad_array_new_length and
function_callv).
The patch seems to reduce bad_alloc and bad_array_new_length calls
considerably:

bad_alloc 380->147
bad_array_new_length 832->128

libstdc++-v3/ChangeLog:

	PR tree-optimization/109442
	* include/bits/stl_vector.h: (vector::size(),
	vector::capacity()): Add __builtin_unreachable call to announce
	that size and capacity are non-negative.

gcc/testsuite/ChangeLog:

	PR tree-optimization/109442
	* g++.dg/tree-ssa/pr109442.C: New test.

gcc/testsuite/g++.dg/tree-ssa/pr109442.C

@@ -1,5 +1,5 @@
 // { dg-do compile { target c++11 } }
-// { dg-options "-O1 -fdump-tree-optimized" }
+// { dg-options "-O2 -fdump-tree-optimized" }
 #include <vector>
 #define T int
 T vat1(std::vector<T> v1) {

libstdc++-v3/include/bits/stl_vector.h

@@ -1114,7 +1114,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return size_type(this->_M_impl._M_finish - this->_M_impl._M_start); }
+      {
+	ptrdiff_t __dif = this->_M_impl._M_finish - this->_M_impl._M_start;
+	if (__dif < 0)
+	   __builtin_unreachable ();
+	return size_type(__dif);
+      }
 
       /**  Returns the size() of the largest possible %vector.  */
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
@@ -1201,8 +1206,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       size_type
       capacity() const _GLIBCXX_NOEXCEPT
       {
-	return size_type(this->_M_impl._M_end_of_storage
-			   - this->_M_impl._M_start);
+	ptrdiff_t __dif = this->_M_impl._M_end_of_storage
+			  - this->_M_impl._M_start;
+	if (__dif < 0)
+	   __builtin_unreachable ();
+	return size_type(__dif);
       }
 
       /**