[FPEnv] Partially implement #pragma STDC FENV_ROUND

This change implements pragma STDC FENV_ROUND, which is introduced by
the extension to standard (TS 18661-1). The pragma is implemented only
in frontend, it sets apprpriate state of FPOptions stored in Sema. Use
of these bits in constant evaluation adn/or code generator is not in the
scope of this change.

Parser issues warning on unsuppored pragma when it encounteres pragma
STDC FENV_ROUND, however it makes syntax checks and updates Sema state
as if the pragma were supported.

Primary purpose of the partial implementation is to facilitate
development of non-default floating poin environment. Previously a
developer cannot set non-default rounding mode in sources, this mades
preparing tests for say constant evaluation  substantially complicated.

Differential Revision: https://reviews.llvm.org/D86921

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