IMPL_RTL_INTERN -> IMPL_RTL_IS_USTRING

Change-Id: I9793f697f14118340bc6db89540fe50ad0b8ccbd
Assert ASCII precond of rtl_uString_newFromAscii/Literal

...and there is no reason for such a precond for rtl_string_newFromLiteral

Change-Id: I27f7217c8db17cef860c37886d0f7e561dc852f8
unoidl: Use appropriate OUString functions on string constants

Change-Id: I6ff256e091770d67f98b1b6b8d447a22fd720d4e