`ir_resolve_str()` bug returns array expression even when when sliced
to a lesser length. Fix is to return array if slice.len == array.len,
otherwise return slice.

Bug report use-case is based on one builtin function. However, at least
the following builtins were exposed to the bug:

    `@byteOffsetOf`
    `@cDefine`
    `@cImport`
    `@cInclude`
    `@cUndef`
    `@compileError`
    `@embedFile`
    `@export`
    `@fieldParentPtr`
    `@hasDecl`
    `@hasField`
    `@import`
    `@unionInit`

closes #3384