[ADT] Rename StringRef case insensitive methods for clarity

Rename functions with the `xx_lower()` names to `xx_insensitive()`.
This was requested during the review of D104218.

Test names and variables in llvm/unittests/ADT/StringRefTest.cpp
that refer to "lower" are renamed to "insensitive" correspondingly.

Unused function aliases with the former method names are left
in place (without any deprecation attributes) for transition purposes.

All references within the monorepo will be changed (with essentially
mechanical changes), and then the old names will be removed in a
later commit.

Also remove the superfluous method names at the start of doxygen
comments, for the methods that are touched here. (There are more
occurrances of this left in other methods though.) Also remove
duplicate doxygen comments from the implementation file.

Differential Revision: https://reviews.llvm.org/D104819