psi2ir: support unsigned integer literals

NB in FE unsigned integer constants are now represented using signed
integer types (e.g., UInt constant actually holds an Int value).
So, in IR so far we also represent unsigned constant literals as
constant values of corresponding signed types, but with corresponding
unsigned type:
0xFFFF_FFFFu becomes 'CONST Int type=kotlin.UInt value=-1'

compiler/testData/ir/irText/expressions/unsignedIntegerLiterals.kt

@@ -0,0 +1,23 @@
+// !LANGUAGE: +InlineClasses
+// !SKIP_METADATA_VERSION_CHECK
+// WITH_UNSIGNED
+
+val testSimpleUIntLiteral = 1u
+
+val testSimpleUIntLiteralWithOverflow = 0xFFFF_FFFFu
+
+val testUByteWithExpectedType: UByte = 1u
+
+val testUShortWithExpectedType: UShort = 1u
+
+val testUIntWithExpectedType: UInt = 1u
+
+val testULongWithExpectedType: ULong = 1u
+
+val testToUByte = 1.toUByte()
+
+val testToUShort = 1.toUShort()
+
+val testToUInt = 1.toUInt()
+
+val testToULong = 1.toULong()