[clang-tidy] performance-* checks: Also allow allow member expressions to be...

[clang-tidy] performance-* checks: Also allow allow member expressions to be used in a const manner.

Until now when determining all the const uses of a VarDecl we only considered
how the variable itself was used. This change extends checking for const usages
of the type's members as well.

This increases the number of true positives for various performance checks that
share the same const usage analysis.

Path by Felix Berger

Reviewed By: njames93, PiotrZSL

Differential Revision: https://reviews.llvm.org/D97567