[libc++] Implement Freestanding

[ldionne]

This issue tracks the work required to make libc++ conforming to the notion of "Freestanding" as specified in the Standard.

At a high level, this means that clang++ -ffreestanding -stdlib=libc++ should work and provide only the Freestanding parts of the Standard. Furthermore, it should require only reasonable functionality from the underlying system, although exactly how that should be achieved still needs to be discussed.

[frederick-vs-ja]

Standard-related issues:

• P1642R11: Freestanding [utilities], [ranges], and [iterators] #105197
• P1855R0: Make <compare> freestanding #100030
• P2013R5: Freestanding Language: Optional ::operator new #105370
• P2198R7: Freestanding Feature-Test Macros and Implementation-Defined Extensions #105368
• P2338R4: Freestanding Library: Character primitives and the C library #105369
• P2407R5: Freestanding Library: Partial Classes #105391
• P2833R2: Freestanding Library: inout expected span #105405
• P2937R0: Freestanding: Remove strtok #105404
• LWG3148: <concepts> should be freestanding #100250
• LWG3753: Clarify entity vs. freestanding entity #105022
• LWG3814: Add freestanding items requested by NB comments #105052
• LWG3841: <version> should not be "all freestanding" #105083
• LWG4076: concat_view should be freestanding #105347