[ASTImporter] Merge ExprBits

Summary:
Some `Expr` classes set up default values for the `ExprBits` of `Stmt`.  These
default values are then overwritten by the parser sometimes.  One example is
`InitListExpr` which sets the value kind to be an rvalue in the ctor.  However,
this bit may change after the `InitListExpr` is created.  There may be other
expressions similar to `InitListExpr` in this sense, thus the safest solution
is to copy the expression bits.

The lack of copying `ExprBits` causes an assertion in the analyzer engine in a
specific case: Since the value kind is not imported, the analyzer engine
believes that the given InitListExpr is an rvalue, thus it creates a
nonloc::CompoundVal instead of creating memory region (as in case of an lvalue
reference).

Reviewers: a_sidorin, r.stahl, xazax.hun, a.sidorin

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D51533

llvm-svn: 341316