Compound literals, enums, et al require const expr

Compound literals, enums, et al require const expr

Summary:
Compound literals,  enums, file-scoped arrays, etc. require their
initializers and size specifiers to be constant. Wrap the initializer
expressions in a ConstantExpr so that we can easily check for this later
on.

Reviewers: rsmith, shafik

Reviewed By: rsmith

Subscribers: cfe-commits, jyknight, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D53921

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346455 91177308-0d34-0410-b5e6-96231b3b80d8