Clean up union_reinterpret_cast()
Clean up union_reinterpret_cast()

Moved duplicated conversion_union out of the const and non-const variants
of union_reinterpret_cast().

Tested:
Verified mathfu builds and all tests pass on Linux.

Bug: 16566707
Change-Id: Ib7ce20e27bcbd5a62b6e74acea8febf70053c4c1

google/mathfu,google/mathfu,google/mathfu