cmTarget: Store strings instead of cmSourceFile* to represent SOURCES.

This will allow the strings to contain generator expressions.

At this point, generator expressions are still not part of the
SOURCES property when it is read.
Genex: Evaluate TARGET_OBJECTS as a normal expression.
cmStringCommand: Add GENEX_STRIP subcommand.

Strip out any generator expressions in the input string.
cmGlobalGenerator: Add interface to call ForceLinkerLanguages

Avoid calling it too early when cmGeneratorTarget instances don't
yet exist.
cmTarget: Include TARGET_OBJECTS genex in target SOURCES property.

Add policy CMP0051 to control this behavior.
cmTarget: Allow any generator expression in SOURCES property.

Remove use of UseObjectLibraries from Makefile and Ninja generators. It
is not needed now because those generators use GetExternalObjects
which already contains the objects from object libraries.

The VS10 generator calls both the UseObjectLibraries and the GetExternalObjects
methods. Ensure that duplicates are not created by skipping objects
from object libraries in handling of GetExternalObjects.

The error message in the BadSourceExpression1 test is now reported
by the generator expression evaluator, so it has different text.
cmComputeTargetDepends: Track object library depends.

Relieve cmGeneratorTarget of that responsibility.
cmGeneratorTarget: Compute consumed object libraries on demand.

Remove up-front object library computation from cmGlobalGenerator.

Adjust tests for message coming from the generator expression
evaluation.
cmTarget: Compute languages from object libraries on demand.
cmGeneratorTarget: Compute target objects on demand

Add a ComputeObjectMapping method to compute the object
names.  It takes mapping to populate as an out-parameter so
that it can be extended in the future with parameters
relevant to generator expression evaluation.

Remove the supporting cmGeneratorTarget::AddObject method. It is
no longer needed as the container member is populated directly.

The ComputeObjectMapping method is called whenever objects are
requested from the cmGeneratorTarget.  Because the Xcode generator
makes no such request, explicitly invoke the method from that
generator so that the logic of checking for bad sources in object
libraries is executed.

In a follow-up, the UseObjectLibraries usage may be replaced by a
true generator expression evaluator for TARGET_OBJECTS. That
will require generators to use cmGeneratorTarget::GetExternalObjects
which is not currently the case for Xcode and VS generators.
Get more information from this test on failure.
Merge topic 'target-sources-refactor' into next

d44b546a Get more information from this test on failure.
7d74e5f9 cmGeneratorTarget: Compute target objects on demand
1e721e01 cmTarget: Compute languages from object libraries on demand.
d60f9ba8 cmGeneratorTarget: Compute consumed object libraries on demand.
e1c764b3 cmComputeTargetDepends: Track object library depends.
3ef2bead cmTarget: Allow any generator expression in SOURCES property.
80f9867c cmTarget: Include TARGET_OBJECTS genex in target SOURCES property.
e52eb7c9 cmGlobalGenerator: Add interface to call ForceLinkerLanguages
013bd822 cmStringCommand: Add GENEX_STRIP subcommand.
77c510dd Genex: Evaluate TARGET_OBJECTS as a normal expression.
82feec59 cmTarget: Store strings instead of cmSourceFile* to represent SOURCES.