[clang-tidy][libc] Add an inline function checker for the libc project.

The new checker checks if inline functions defined in header files are
tagged with the LIBC_INLINE macro. See https://libc.llvm.org/code_style.html
for more information about this macro.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D142592