[cxxmodules] Fix the infinite loop in rootcling when building GenVector.
[cxxmodules] Fix the infinite loop in rootcling when building GenVector.

Adding Math/Math.h to the MathCore module makes the generation of GenVector
dictionary hand due to a bug in rootcling/clang. The redeclaration chain of
'abs' shadow declaration (from stl math.h) is broken.

In general there are a few remaining issues with stl's cmath, math.h and
libc math.h header files. We cannot yet reliably put them in the modulemaps.
This causes the current issue: we end up with content duplication which
triggers a bugs in clang.

karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,karies/root