Make $<LINK_ONLY> available to projects (#14751)

Previously this generator expression was used internally by the
target_link_libraries command to honor private linking requirements of
static libraries in their INTERFACE_LINK_LIBRARIES.  Remove the check
that limits $<LINK_ONLY> to this use case to make it available for
project code to use too.
Honor $<LINK_ONLY> when checking interface properties

Callers of cmTarget::GetLinkImplementationClosure are interested in the
set of targets whose interface properties propagate to the current
target.  This excludes targets guarded by $<LINK_ONLY>.

Teach the CompatibleInterface test to cover suppression of interface
compatibility tests with $<LINK_ONLY>.  Although this is not recommended
in practice, it is a way of covering the above behavior.
cmTarget: Make GetLink*Libraries methods safer to use

Split the library lists out of LinkImplementation and LinkInterface into
LinkImplementationLibraries and LinkInterfaceLibraries parent classes,
respectively.  Return these from GetLinkImplementationLibraries and
GetLinkInterfaceLibraries, respectively, so that callers cannot access
parts of the structures that have not been populated.
Refactor system include annotation propagation

Since commit v3.0.0-rc1~174^2~1 (cmTarget: Fix system include annotation
propagation, 2014-01-01) the cmGeneratorTarget::IsSystemIncludeDirectory
method needs to collect all targets that might provide
INTERFACE_(|SYSTEM)_INCLUDE_DIRECTORIES for the current target.  We now
have cmTarget::GetLinkImplementationClosure to provide this, so use it.
cmTarget: Drop GetTransitivePropertyTargets method

Inline the implementation at the only remaining call site.
Genex: Simplify TARGET_PROPERTY transitive lookup

In cmGeneratorExpressionEvaluator, make getLinkedTargetsContent a
template so it can traverse over either the Libraries in a cmTarget
LinkImplementationLibraries or a cmTarget LinkInterfaceLibraries.  This
also avoids creating a separate vector<cmTarget*>.
Genex: Avoid repeated search of transitive property whitelist

In cmGeneratorExpressionEvaluator, avoid searching through the list of
transitive interface property names repeatedly during evaluation of
TargetPropertyNode.  Simply record the results of the first search for
later re-use.
Genex: Refactor empty element strip

In cmGeneratorExpressionEvaluator, teach getLinkedTargetsContent to call
cmGeneratorExpression::StripEmptyListElements to transform its return
value so that callers do not have to do so.
Genex: Adjust code layout slightly
cmTarget: Refactor GetLinkImplementationClosure internals

Store the 'Done' flag directly in each map entry instead of using a
separate map.
cmTarget: Cache compatible interface property sets

Replace isLinkDependentProperty with a CompatibleInterfaces structure
that records all the compatible interface properties in a set for each
type.  This avoids repeatedly traversing the link implementation closure
and asking every target for its compatible interface properties.
Help: Add notes for topic 'generalize-LINK_ONLY'
cmcurl: Fix a build failure with the Cray compiler on Linux (#15026)

The error is caused by a workaround for UNICOS.  The workaround
incorrectly uses the _CRAYC macro to determine if building for
UNICOS.  This macro will always be defined for the Cray C and
C++ compiler regardless of the target platform, even when
building for Linux.  The correct macro should be _CRAY, which as
per Cray documentation is only defined for UNICOS targeted
builds.
Merge topic 'compile-with-cray'

fd63722d cmcurl: Fix a build failure with the Cray compiler on Linux (#15026)
Merge topic 'generalize-LINK_ONLY'

10c5c82c Help: Add notes for topic 'generalize-LINK_ONLY'
6e7e881c Honor $<LINK_ONLY> when checking interface properties
0400cd5d Make $<LINK_ONLY> available to projects (#14751)
Merge topic 'refactor-link-internals'

c45dd669 cmTarget: Cache compatible interface property sets
89095514 cmTarget: Refactor GetLinkImplementationClosure internals
9d72df45 Genex: Adjust code layout slightly
d5f0743d Genex: Refactor empty element strip
60bafeb6 Genex: Avoid repeated search of transitive property whitelist
8cb91054 Genex: Simplify TARGET_PROPERTY transitive lookup
0a8fbac1 cmTarget: Drop GetTransitivePropertyTargets method
fb3518dc Refactor system include annotation propagation
535fd6ce cmTarget: Make GetLink*Libraries methods safer to use