[Frontend][OpenMP] Sort all the things in OMP.td, NFC

The file OMP.td is becoming tedious to update by hand due to the
seemingly random ordering of various items in it. This patch brings
order to it by sorting most of the contents.

The clause definitions are sorted alphabetically with respect to the
spelling of the clause.[1]

The directive definitions are split into two leaf directives and
compound directives.[2] Within each, definitions are sorted alphabetically
with respect to the spelling, with the exception that "end xyz"
directives are placed immediately following the definition of "xyz".[3]

Within each directive definition, the lists of clauses are also sorted
alphabetically.

[1] All spellings are made of lowercase letters, _, or space. Ordering
between non-letters follow the order assumed by `sort` utility.
[2] Compound directives refer to the consituent leaf directives, hence
the leaf definitions must come first.
[3] Some of the "end xyz" directives have properties derived from the
corresponding "xyz" directive. This exception guarantees that "xyz"
precedes the "end xyz".