Cache context-independent includes on evaluation.

Generator expressions whose output depends on the configuration
now record that fact. The GetIncludeDirectories method can use
that result to cache the include directories for later calls.

GetIncludeDirectories is called multiple times for a target
for each configuration, so this should restore performance for
multi-config generators.
Merge topic 'tll-includes-defines' into next

e48d842 Cache context-independent includes on evaluation.