[libc++] Fix initialization of __fill_

`basic_ios` delays initialization of `__fill_` to `widen(' ')` until `fill()` is called. But, `fill(char_type)` is missing this logic, so the fill character does not get initialized to whitespace if `fill(char_type)` is called first. This patch adds this logic to `fill(char_type)`.

Reviewed By: #libc, ldionne, Quuxplusone

Differential Revision: https://reviews.llvm.org/D120751

libcxx/include/ios

@@ -781,6 +781,8 @@ inline _LIBCPP_INLINE_VISIBILITY
 _CharT
 basic_ios<_CharT, _Traits>::fill(char_type __ch)
 {
+    if (traits_type::eq_int_type(traits_type::eof(), __fill_))
+        __fill_ = widen(' ');
     char_type __r = __fill_;
     __fill_ = __ch;
     return __r;

libcxx/test/std/input.output/iostreams.base/ios/basic.ios.members/fill_char_type.pass.cpp

@@ -20,7 +20,6 @@
 int main(int, char**)
 {
     std::ios ios(0);
-    assert(ios.fill() == ' ');
     char c = ios.fill('*');
     assert(c == ' ');
     assert(ios.fill() == '*');