[FFI][LLVM] Fix compilation errors with clang20

[cbalint13]

Tiny fixes for the refactored FFI backend & LLVM math macros.

• FFI related compilation error:

# make -j8
{...}
In file included from tvm-0.20-build/tvm/ffi/include/tvm/ffi/container/array.h:29:
In file included from tvm-0.20-build/tvm/ffi/include/tvm/ffi/any.h:27:
tvm-0.20-build/tvm/ffi/include/tvm/ffi/string.h:214:10: error: field has incomplete type 'String'
  214 |   String(nullptr_t) = delete;  // NOLINT(*)
{...}
# rpm -q clang
clang-20.1.2-3.fc42.riscv64

• Inclusion fix for MSWIN (introduced @ Add registion for the operator asin and acos in llvm #17945):

intrin_rule_llvm.cc(185,48): error C2065: 'M_PI': undeclared identifier

[cbalint13]

cc @Hzfengsy @Lunderberg @mshr-h
cc @tqchen