[AST] Attempt to fix buildbot warnings + appease MSVC; NFCI
[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Additionally, I remember hearing (though my knowledge may be outdated)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.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@329652 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 89dbe05bfd0704a71ad1713709bda144a31ac1ef)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang