All generators now support detection of the full path to the compiler, so
require it to be so.  This will allow CMake<LANG>Information.cmake and
other logic to assume the full path to the compiler tool is available.

The Makefile generators already rejected CMAKE_<LANG>_COMPILER values
that did not name an existing compiler.  Extend this error message to
all generators, make it occur as early as possible, and improve the
message with advice about how to set the compiler.  If the full path to
the compiler is not known, finish enabling languages with a fatal error
so configuration does not continue.

For now, allow the RC language compiler to not be a full path.  Later we
will need to detect the full path to "rc" under the VS IDE.

Add a RunCMake.CompilerNotFound test to cover failure cases.
Fix the RunCMake.CompilerChange test EmptyCompiler case to work
when configuration does not continue past enable_language.

This new expression allows checking how a policy was set when a target
was created.  That information is only recorded for a subset of policies,
so a whitelist is used.

Add generator expressions that combine and use boolean test results:

 $<0:...>         = empty string (ignores "...")
 $<1:...>         = content of "..."
 $<AND:?[,?]...>  = '1' if all '?' are '1', else '0'
 $<OR:?[,?]...>   = '0' if all '?' are '0', else '1'
 $<NOT:?>         = '0' if '?' is '1', else '1'

These will be useful to evaluate (future) boolean query expressions and
condition content on the results.  Include tests and documentation.

Since commit e1409ac5 (Support building shared libraries or modules
without soname, 2012-04-22) CMake crashes on the code

 add_library(foo SHARED foo.nolang)

because the logic to lookup the language's soname flag was moved from
cmTarget::GetLibraryNames to cmMakefile::GetSONameFlag without its check
for a NULL language.  Restore the check for NULL.

Add RunCMake.Languages test to cover language error cases like this one.

Add "RunCMake.ObjectLibrary" test to verify that unsupported use cases
are rejected with errors:

* An OBJECT library may not reference another object library
* An OBJECT library may not be referenced in target_link_libraries
* An OBJECT library may not contain non-compiling sources
* An OBJECT library may not have pre/post build/link commands
* An OBJECT library may not be installed, exported, or imported

Also verify that invalid $<TARGET_OBJECTS:...> expressions are
diagnosed.

The CMakeCommands.build_command test performs output/error checking
so move it over to RunCMake to re-use the generalized infrastrucure.
This is the only test left using Tests/CMakeCommands/CMakeLists.txt
so remove it.

Add a "CMakeCommands.find_package" test to run CMake on a bunch of cases
in which find_package fails.  Check that the process return code and
warning/error messages are as expected.  Record expected test output in
corresponding files for reference by the check.  These files will also
serve as a reference for the message text in each case.