[libc++] Remove libc++'s <float.h> (#175849)
The existence of <float.h> in libc++ leads to difficulties with modules,
especially when it comes to supporting the various __need macros
supported by the Clang builtin headers.
Fortunately, libc++'s <float.h> only defines FLT_EVAL_METHOD and
DECIMAL_DIG, which are already defined by the Clang builtin headers in
C++11 mode. Hence, removing the header entirely should have no impact,
apart from not defining FLT_EVAL_METHOD and DECIMAL_DIG in C++03 mode.
However, that is arguably something that libc++ shouldn't be doing: if
the underlying C library doesn't want to provide these definitions, then
libc++ shouldn't force it to.
