[APFloat] Remove workaround for old clang.

[craig.topper]

Summary

The comment says this is for clang 3.3. I believe our build
requirements are newer than that so we can remove this.

[nikic]

LG, minimum clang is 5.0.

[RKSimon]

LGTM

llvm/include/llvm/ADT/APFloat.h

class APFloat : public APFloatBase {
   void makeSmallest(bool Neg) {
     APFLOAT_DISPATCH_ON_SEMANTICS(makeSmallest(Neg));
   }
 
   void makeSmallestNormalized(bool Neg) {
     APFLOAT_DISPATCH_ON_SEMANTICS(makeSmallestNormalized(Neg));
   }
 
-  // FIXME: This is due to clang 3.3 (or older version) always checks for the
-  // default constructor in an array aggregate initialization, even if no
-  // elements in the array is default initialized.
-  APFloat() : U(IEEEdouble()) {
-    llvm_unreachable("This is a workaround for old clang.");
-  }
-
   explicit APFloat(IEEEFloat F, const fltSemantics &S) : U(std::move(F), S) {}
   explicit APFloat(DoubleAPFloat F, const fltSemantics &S)
       : U(std::move(F), S) {}
 
   cmpResult compareAbsoluteValue(const APFloat &RHS) const {
     assert(&getSemantics() == &RHS.getSemantics() &&
            "Should only compare APFloats with the same semantics");
     if (usesLayout<IEEEFloat>(getSemantics()))