[libcxx] Initialize vcruntime __std_exception_data in the exception copy ctor

This fixes failures in a number of tests, in the
clang-cl-no-vcruntime configuration (where libcxx provides dummy,
no-op replacements of some vcruntime base exception classes), if
building with optimization enabled.

Previously, with optimization enabled, the compiler concluded that
these fields would be uninitialized at the points of asserts in
the tests.

This fixes the following tests in this configuration:

    llvm-libc++-shared-no-vcruntime-clangcl.cfg.in :: std/language.support/support.dynamic/alloc.errors/bad.alloc/bad_alloc.pass.cpp
    llvm-libc++-shared-no-vcruntime-clangcl.cfg.in :: std/language.support/support.dynamic/alloc.errors/new.badlength/bad_array_new_length.pass.cpp
    llvm-libc++-shared-no-vcruntime-clangcl.cfg.in :: std/language.support/support.exception/bad.exception/bad_exception.pass.cpp
    llvm-libc++-shared-no-vcruntime-clangcl.cfg.in :: std/language.support/support.exception/exception/exception.pass.cpp
    llvm-libc++-shared-no-vcruntime-clangcl.cfg.in :: std/language.support/support.rtti/bad.cast/bad_cast.pass.cpp
    llvm-libc++-shared-no-vcruntime-clangcl.cfg.in :: std/language.support/support.rtti/bad.typeid/bad_typeid.pass.cpp

Author: mstorsjo

libcxx/include/__exception/exception.h

@@ -48,7 +48,7 @@ class exception { // base of all library exceptions
     __data_._DoFree = true;
   }
 
-  exception(exception const&) _NOEXCEPT {}
+  exception(exception const&) _NOEXCEPT : __data_() {}
 
   exception& operator=(exception const&) _NOEXCEPT { return *this; }