[libc] Rename FEnv.h and refactor subsequent files

Because Windows's pathnames are not case sensitive,
to avoid include conflicts between our header file FEnv.h and the
one from the C Standard library, <fenv.h>, the prior file was renamed.
The motive for the relabel came to fix this include error in
TestHelpers.cpp since a conflict arose with a file in the same
directory when #include <fenv.h> was being used.

Reviewed By: sivachandra, aeubanks

Differential Revision: https://reviews.llvm.org/D106470