[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

llvm-svn: 315234
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 492d7134f3bdd76415d8e7b20a4f1c9a42b85e44
diff --git a/include/exception b/include/exception
index ca2eaf5..79bd6ac 100644
--- a/include/exception
+++ b/include/exception
@@ -82,7 +82,7 @@
 #include <cstdlib>
 #include <type_traits>
 
-#if defined(_LIBCPP_ABI_MICROSOFT)
+#if defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_NO_VCRUNTIME)
 #include <vcruntime_exception.h>
 #endif
 
@@ -93,7 +93,7 @@
 namespace std  // purposefully not using versioning namespace
 {
 
-#if !defined(_LIBCPP_ABI_MICROSOFT)
+#if !defined(_LIBCPP_ABI_MICROSOFT) || defined(_LIBCPP_NO_VCRUNTIME)
 class _LIBCPP_EXCEPTION_ABI exception
 {
 public:
@@ -110,7 +110,7 @@
     virtual ~bad_exception() _NOEXCEPT;
     virtual const char* what() const _NOEXCEPT;
 };
-#endif // !_LIBCPP_ABI_MICROSOFT
+#endif // !_LIBCPP_ABI_MICROSOFT || _LIBCPP_NO_VCRUNTIME
 
 #if _LIBCPP_STD_VER <= 14 \
     || defined(_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS) \