Fix a bug in APFloat.cpp: declare APFloat after fltSemantics it

uses. APFloat::convert() takes the pointer to the fltSemantics
variable, which is later accessed it in ~APFloat() desctructor.
That is, semantics must still be alive at the moment we delete
APFloat.

Found by experimental AddressSanitizer use-after-scope checker.

llvm-svn: 169047