[Support] Remove redundant declarations (NFC) (#165971)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Support/APFloat.cpp

@@ -269,12 +269,6 @@ bool APFloatBase::isRepresentableBy(const fltSemantics &A,
          A.precision <= B.precision;
 }
 
-constexpr RoundingMode APFloatBase::rmNearestTiesToEven;
-constexpr RoundingMode APFloatBase::rmTowardPositive;
-constexpr RoundingMode APFloatBase::rmTowardNegative;
-constexpr RoundingMode APFloatBase::rmTowardZero;
-constexpr RoundingMode APFloatBase::rmNearestTiesToAway;
-
 /* A tight upper bound on number of parts required to hold the value
    pow(5, power) is