[ELF] Fix lld build on Windows/MinGW

The patch in https://reviews.llvm.org/D64077 causes a build failure
because both the Defined and SharedSymbol classes are bigger than 80
bytes on MinGW 8.

This patch fixes this build failure by changing the type of the
bitfields. It is a similar change to the bitfield changes in
https://reviews.llvm.org/D64238, but instead of changing to bool I
decided to use uint8_t because one of the bitfields takes up two bits
instead of one.

Note: the patch is slightly different from the one reviewed in
Phabricator, but it is a trivial change to align it with LLVM master
instead of LLVM 9. Also, it passes all lld tests.

Differential Revision: https://reviews.llvm.org/D70266