libc++ currently relies on undefined initialization order of global
initializers when using gcc:
1. __start_std_streams in iostream.cpp calls locale:🆔:_init, which assigns
an id to each locale::facet in an initializer
2. Every facet has a static locale::id id, whose constructor sets the facet's
id to 0
If 2 runs after 1, it clobbers the facet's assigned consecutive id, causing
exceptions to be thrown when e.g. running code like "cout << endl".
To fix this, let _LIBCPP_CONSTEXPR evaluate to "constexpr" instead of nothing
with gcc. locale::id's constructor is marked _LIBCPP_CONSTEXPR, which ensures
that it won't get an initializer that could potentially run after the
iostream.cpp initializer. (This remains broken when building with msvc.)
Also switch constexpr-specific code in bitset to use __SIZEOF_SIZE_T__ instead
of __SIZE_WIDTH__, because gcc doesn't define the latter.
Pair-programmed/debugged with Dana Jansens.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210188 91177308-0d34-0410-b5e6-96231b3b80d8
0211e86a5e