[TableGen] Use a SmallVector for Record::Values to avoid debug iterators

Debug iterators are valuable so we don't want to turn them off
completely. However, llvm-tblgen is critical to build speed, so we can
skip them here.

Regenerating X86GenSubtargetInfo.inc in a clang-cl self-host debug build
now takes 39s instead of 1m29s.

Helps PR28222

llvm-svn: 274288