[clang] Fix compilation with GCC < 8 for MinGW

GCC 7 and earlier, when targeting MinGW, seems to have a bug in
layout/size of bitfield structs if they contain a nested enum,
making the size of the struct 8 bytes, while we have a static assert
requiring it to be 4 bytes or less.

While this clearly is a GCC bug, the workaround (moving the enum out
of the bitfield) also is very nonintrusive and matches other existing
enums there.

Differential Revision: https://reviews.llvm.org/D71650