cmLocalCommonGenerator: Save CMAKE_BUILD_TYPE on construction
Copy the value to our ConfigName member on construction to ensure it
is available even to code paths that run before Generate. We once
needed to delay this lookup until Generate because the local
generators were at one time created before Configure. Now they are
created at generate time which is late enough to expect
CMAKE_BUILD_TYPE to be available.
Without this, cmGlobalUnixMakefileGenerator3::WriteConvenienceRules
causes use of ConfigName before it is populated which breaks use of
source files that depend on the $<CONFIG> generator expression.
Fixes: #16889 (closed)