::Form -> TString::Form(at)

(cherry picked from commit 97cd75e145d8e9d934d2493acb42b585ca554cb9)

Conflicts:
	core/winnt/src/TWinNTSystem.cxx
	io/io/src/TDirectoryFile.cxx
Remove help for builtin-llvm: it is meaningless for non-devs.
Linux 32bit: expose "-msse" to ROOT build clients e.g. roottest.