[DIrectX backend] emit metadata for entry.

New named metadata "dx.entryPoints" is added to save all entries.

Each entry is in format of
!{ptr to function, name, signature, resource table, extra}

For compute shader, the extra will save num of threads in format of {i32 x, i32 y, i32 z}

For library profile, an empty entry will be added to save the resource table for the library.

Signature and resource table metadata is not generated yet.

Differential Revision: https://reviews.llvm.org/D131807