Revert "[Lex] Warn when defining or undefining any builtin macro"

This reverts commit 22e3f587.
Breaks check-clang on arm, see https://reviews.llvm.org/D144654#4349954

Also reverts follow-up "[AArch64] Don't redefine _LP64 and __LP64__"

This reverts commit e55d52cd.