[Clang] Reset FP options before function instantiations

Previously function template instantiations occurred with FP options
that were in effect at the end of translation unit. It was a problem
for late template parsing as these FP options were used as attributes of
AST nodes and may result in crash. To fix it FP options are set to the
state of the point of template definition.

Differential Revision: https://reviews.llvm.org/D143241