c++: fix error reporting routines re-entered ICE [PR110175]

c++: fix error reporting routines re-entered ICE [PR110175]

Here we get the "error reporting routines re-entered" ICE because
of an unguarded use of warning_at.  While at it, I added a check
for a warning_at just above it.

	PR c++/110175

gcc/cp/ChangeLog:

	* typeck.cc (cp_build_unary_op): Check tf_warning before warning.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/decltype-110175.C: New test.