This is a fairly mechanical change, it just moves each algorithm into its own header. This is a NFC.
Note: during this change, I burned down all the includes, so this follows "include only and exactly what you use."
Differential Revision: https://reviews.llvm.org/D103583
This reverts commit 924ea3bb53 *again*, this time because it broke the
LLDB build with modules. We need to figure out what's up with the libc++
modules build once and for all.
Differential Revision: https://reviews.llvm.org/D103369