Xcode: Fix compiler id detection for iOS tools (#15214)

Since commit 0cce556b (Xcode: Use sysroot and deployment target to
identify compiler, 2014-04-29) our compiler id detection project uses
the target platform SDK in case Xcode selects a different compiler based
on it.  Now the compiler id project actually compiles with the target
compiler and SDK when cross-compiling.

The iOS tools do not support the 'com.apple.product-type.tool' product
type we use in our compiler id detection project.  When targeting
iPhone, use product type 'com.apple.product-type.bundle.unit-test'
instead.
Xcode: Fix compiler id detection when code signing is required

The iOS product type 'com.apple.package-type.bundle.unit-test' requires
code signing on Xcode 6.  Other iOS target types do too.  Until CMake
learns to add the CODE_SIGN_IDENTITY build attribute itself, toolchain
files can set CMAKE_XCODE_ATTRIBUTE_CODE_SIGN_IDENTITY to tell the Xcode
generator to add the attribute.  Teach CMakeDetermineCompilerId to
recognize this variable and add the CODE_SIGN_IDENTITY build attribute
to the compiler id project.
Merge topic 'xcode-ios-compiler-id' into next

b91020f6 Xcode: Fix compiler id detection when code signing is required
c48f6e12 Xcode: Fix compiler id detection for iOS tools (#15214)