[AST] Attempt to fix buildbot warnings + appease MSVC; NFCI

GCC 4.8.4 on a bot was warning about `ArgPassingKind` not fitting in
`ArgPassingRestrictions`, which appears to be incorrect, since
`ArgPassingKind` only has three potential values:

"warning: 'clang::RecordDecl::ArgPassingRestrictions' is too small to
hold all values of 'enum clang::RecordDecl::ArgPassingKind'"

Additionally, I remember hearing (though my knowledge may be outdated)
that MSVC won't merge adjacent bitfields if their types are different.

Try to fix both issues by turning these into `uint8_t`s.

llvm-svn: 329652