Add support for TPDE-LLVM: 10x faster unoptimized compilation

[JohanEngelen]

TPDE-LLVM is a TPDE-based LLVM back-end focusing on fast compilation targeting x86-64 and AArch64. Typically, compile times are 10–20x faster than the LLVM -O0 back-end with similar execution time, code size is ~10-30% larger for -O0 IR and similar for -O1 IR. The focus is on supporting a commonly used subset of LLVM-IR and target platforms efficiently, therefore many IR features are not supported – in such cases, the intention is to fall back to the full LLVM back-end. Code generated by Clang (-O0/-O1) will typically compile; -O2 and higher will typically fail due to unsupported vector operations.

See https://docs.tpde.org/tpde-llvm-main.html

Seems worth a try and relatively simple to implement (optional) support!

[kinke]

Unsupported features currently include:

* Targets other than x86-64-v1/AArch64 (ARMv8.1) (Linux) ELF.
* Code models other than Small-PIC.
* Scalar types: integer types larger than i64 except i128 (i128 is supported), pointers with non-zero address space, half, bfloat, ppc_fp128, x86_fp80, x86_amx. Code with x86-64 long double needs to be compiled with -mlong-double-64.
* Vectors: types that are not directly legal on the target (e.g., <32 x i8> on x86-64); icmp/fcmp; pointer element type; getelementptr with vector types; select with vector predicate, integer extension/truncation,
* select aggregate type other than {i64, i64}.
* bitcast larger than 64 bit.
* Atomic operations might use a stronger consistency than required (e.g., always seqcst for atomicrmw).
* Calling conventions other than the C calling convention (SysV on x86-64, AAPCS on AArch64).
* fp128: fneg, fcmp one/ueq, many intrinsics.
* Computed goto (blockaddress, indirectbr).
* landingpad with non-empty filter clause.
* Many intrinsics, and some intrinsics are only implemented for commonly used types (e.g., llvm.cttz only for 8/16/32/64-bit).
* IFuncs.
* Various forms of constant expressions in global initializers.
* Non-empty inline assembly.
* Full asynchronous unwind info (frame info only correct in prologue and at call sites).
* Several corner cases that we didn't encounter so far.

So Linux x86_64 and AArch64 only for now. Without x87 (x86_fp80) support, so no real on x86_64, and only limited support for Linux-AArch64's fp128 real. Coupled with incomplete intrinsics support and no support for (non-empty :D) inline asm, there's most likely a long way to go to get druntime to compile.