[libc++] Support Microsoft ABI without vcruntime headers

The vcruntime headers are hairy and clash with both libc++ headers
themselves and other libraries. libc++ normally deals with the clashes
by deferring to the vcruntime headers and silencing its own definitions,
but for clients which don't want to depend on vcruntime headers, it's
desirable to support the opposite, i.e. have libc++ provide its own
definitions.

Certain operator new/delete replacement scenarios are not currently
supported in this mode, which requires some tests to be marked XFAIL.
The added documentation has more details.

Differential Revision: https://reviews.llvm.org/D38522

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315234 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.dynamic/new.delete/new.delete.array/new_array_nothrow_replace.pass.cpp

@@ -10,6 +10,7 @@
 // test operator new [] nothrow by replacing only operator new
 
 // UNSUPPORTED: sanitizer-new-delete
+// XFAIL: libcpp-no-vcruntime
 
 #include <new>
 #include <cstddef>

test/std/language.support/support.dynamic/new.delete/new.delete.array/new_array_replace.pass.cpp

@@ -10,6 +10,7 @@
 // test operator new[] replacement by replacing only operator new
 
 // UNSUPPORTED: sanitizer-new-delete
+// XFAIL: libcpp-no-vcruntime
 
 
 #include <new>

test/std/language.support/support.dynamic/new.delete/new.delete.single/new_nothrow_replace.pass.cpp

@@ -10,6 +10,7 @@
 // test operator new nothrow by replacing only operator new
 
 // UNSUPPORTED: sanitizer-new-delete
+// XFAIL: libcpp-no-vcruntime
 
 #include <new>
 #include <cstddef>