[DebugInfo] Fix DwarfExpression::addConstantFP for float on big-endian

The byte swapping, when dealing with 4 byte (float) FP constants
in DwarfExpression::addConstantFP, added in commit ef8992b9
was not correct. It always performed byte swapping using an
uint64_t value. When dealing with 4 byte values the 4 interesting
bytes ended up in the big end of the uint64_t, but later we emitted
the 4 bytes at the little end. So we ended up with zeroes being
emitted and faulty debug information.

This patch simplifies things a bit, IMHO. Using the APInt
representation throughout the function, instead of looking at
the internal representation using getRawBytes and without using
reinterpret_cast etc. And using API.byteSwap() should result in
correct byte swapping independent of APInt being 4 or 8 bytes.

Differential Revision: https://reviews.llvm.org/D86272