[AST] Pack the unsigned of PackExpansionType into Type
[AST] Pack the unsigned of PackExpansionType into Type

The bit-fields of `Type` have enough space for
the `unsigned NumExpansions` of `PackExpansionType`

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D50711



git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@339789 91177308-0d34-0410-b5e6-96231b3b80d8

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