[libclang] For getDeclFromExpr in CIndex.cpp, associate the decl of

[libclang] For getDeclFromExpr in CIndex.cpp, associate the decl of
a DeclRefExpr, MemberExpr, etc. with a CastExpr if it is ImplicitCast,
since the implicit cast is the one that is invisible in source code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139547 91177308-0d34-0410-b5e6-96231b3b80d8