[clang-tidy] Optimize misc-confusable-identifiers

This is final optimization for this check. Main
improvements comes from changing a logic order
in mayShadow function, to first validate result
of mayShadowImpl, then search primary context in
a vectors. Secondary improvement comes from excluding
all implicit code by using TK_IgnoreUnlessSpelledInSource.
All other changes are just cosmetic improvements.

Tested on Cataclysm-DDA open source project, result in
check execution time reduction from 3682 seconds to
100 seconds (~0.25s per TU). That's 97.2% reduction for
this change alone. Resulting in cumulative improvement for
this check around -99.6%, finally bringing this check
into a cheap category.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D151594