[clang-tidy] performance-unnecessary-copy-initialization: Prevent false positives...

[clang-tidy] performance-unnecessary-copy-initialization: Prevent false positives when dependent variable is modified.

Extend the check to not only look at the variable the unnecessarily copied
variable is initialized from, but also ensure that any variable the old variable
references is not modified.

Extend DeclRefExprUtils to also count references and pointers to const assigned
from the DeclRef we check for const usages.

Reviewed-by: aaron.ballman
Differential Revision: https://reviews.llvm.org/D91893