[NFC][Object] Include header `BitcodeReader.h` instead of using forward...

[NFC][Object] Include header `BitcodeReader.h` instead of using forward declaration for BitcodeModule

`BitcodeModule` is used as element of a vector in `IRSymtabFile`, while in the
header there is only a forward declaration. It will work if the header `BitcodeReader.h`
is included before including `IRObjectFile.h`. However, it is not always the case,
causing compilation error. This patch simply includes the header and remove the
forward declaration.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D139556