cmTarget: Use string API to add sources to cmTarget objects.

Continue to call GetOrCreateSource where necessary to create
cmSourceFile objects which have the GENERATED attribute set.
cmTarget: Remove AddSourceFile method

It is no longer used.
cmSourceFileLocation: Normalize compared directories.

Otherwise Matches() ends up doing a comparison of
the directories

 /path/to/dir/subdir/..

and

 /path/to/dir

as strings and not matching where it should.
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.

Similarly, fix Xcode object handling by skipping external objects from
OBJECT_LIBRARY usage as appropriate.

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.
Genex: Only evaluate TARGET_OBJECTS to determine target sources.

The output of this expression may contain macros for IDEs to replace
such as $(Configuration), $(CURRENT_ARCH) etc.  To avoid generating
content which is not usable in other contexts, report an error if
there is an attempt to use it in other contexts.

This commit may be reverted in the future if a solution to the
above difference is implemented.
Merge topic 'target-sources-refactor' into next

b411d37e Genex: Only evaluate TARGET_OBJECTS to determine target sources.
bc3249a3 cmGeneratorTarget: Compute target objects on demand
a1b64837 cmTarget: Compute languages from object libraries on demand.
d136c68c cmGeneratorTarget: Compute consumed object libraries on demand.
79b58575 cmComputeTargetDepends: Track object library depends.
6ff59169 cmTarget: Allow any generator expression in SOURCES property.
2714dd1f cmTarget: Include TARGET_OBJECTS genex in target SOURCES property.
fc3fca11 cmGlobalGenerator: Add interface to call ForceLinkerLanguages
49a36fab cmStringCommand: Add GENEX_STRIP subcommand.
c1c54024 Genex: Evaluate TARGET_OBJECTS as a normal expression.
8cb3cb72 cmTarget: Store strings instead of cmSourceFile* to represent SOURCES.
e081db39 cmSourceFileLocation: Normalize compared directories.
53fc5756 cmTarget: Remove AddSourceFile method
9cfffc0d cmTarget: Use string API to add sources to cmTarget objects.