[D] Fix crash when debug expression enabled.

[D] Fix crash when debug expression enabled.

While casting works as expected with expression debugging turned off,
this seems to be an indication that the D language parser function is
doing something wrong in the building of the expression.

Without changing the grammar, using UNOP_CAST_TYPE is the right thing to
do here, as the TypeExp handler has already wrapped the type around a
pair of OP_TYPE opcodes.

gdb/ChangeLog:

	* d-exp.y (CastExpression): Emit UNOP_CAST_TYPE.

gdb/testsuite/ChangeLog:

	* gdb.dlang/debug-expr.exp: New file.