Support/reg*: Roll some non-modular headers into their singular uses

Support/reg*: Roll some non-modular headers into their singular uses

These headers have static variables in them, which would easily create
ODR violations if the header was included in another header, and the
constants were used by an inline function, for example.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316706 91177308-0d34-0410-b5e6-96231b3b80d8