Fix gdb.base/infcall-nested-structs-c++.exp with Clang

Fix gdb.base/infcall-nested-structs-c++.exp with Clang

gdb.base/infcall-nested-structs-c++.exp failed to build using Clang
with many variations on the following error:

  gdb compile failed, /gdbtest/src/gdb/testsuite/gdb.base/infcall-nested-structs.c:207:46:
    warning: self-comparison always evaluates to true [-Wtautological-compare]

This commit builds this testcase with -Wno-tautological-compare when
using Clang, to avoid this failure.

gdb/testsuite/ChangeLog:

	* gdb.base/infcall-nested-structs.exp.tcl: Add
	additional_flags=-Wno-tautological-compare for C++
	tests when compiling using Clang.