Address post-commit review comments after r328731. NFC.

- Define a function (canPassInRegisters) that determines whether a
record can be passed in registers based on language rules and
target-specific ABI rules.

- Set flag RecordDecl::ParamDestroyedInCallee to true in MSVC mode and
remove ASTContext::isParamDestroyedInCallee, which is no longer needed.

- Use the same type (unsigned) for RecordDecl's bit-field members.

For more background, see the following discussions that took place on
cfe-commits.

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20180326/223498.html
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20180402/223688.html
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20180409/224754.html
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20180423/226494.html
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20180507/227647.html

llvm-svn: 332397