[clang][ExprConstant] Improve error message of compound assignment against uninitialized object

BEFORE this patch, compound assignment operator against uninitialized object such as uninit += 1 was diagnosed as subexpression not valid
This patch clarifies the reason for the error by saying that uninit is an uninitialized object.

Fixes https://github.com/llvm/llvm-project/issues/51536

Reviewed By: shafik, tbaeder
Differential Revision: https://reviews.llvm.org/D157855