[clang-tidy] readability-identifier-naming: fix hungarian enum prefix in C

When checking a C file, enum tags are of type `int` instead of the enum
type as in C++.
So the checker was using `i` as the prefix for enum tags instead of main
letters of the enum type name (for example `rt` for `enum REV_TYPE`).

This commit fixes this divergence and makes the behavior the same between
C and C++ files.

For example, `enum REV_TYPE { rtRevValid };` won't be reported as badly
named when in a C file. Previously, it would have proposed a rename to
`iRevValid`.

This commit also add a file to test the hungarian notation checker with C
files.
The test file was made from identifier-naming-hungarian-notation.cpp
with C++-only features removed.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D144912