...to use single ASCII character literals "more directly" in the OUString API
(instead of having to go via an intermediary OUString ctor call).  Especially
useful for character literals that are defined as const variables or via macros
("direct" uses of character literals in the OUString API can often simply be
replaced with single-character string literals, for improved readability).

(The functions overloaded for OUStringLiteral1 are those that are actually used
by the existing LO code; more could potentially be added.  The asymmetry in the
operator ==/!= parameter types is by design, though---writing code like

  'x' == s

is an abomination that shall not be abetted.)

Change-Id: Ic5264714be7439eed56b5dfca6ccaee277306f1f