[AMDGPU] Start documenting calling conventions. NFC

Add a section to AMDGPUUsage.rst about calling conventions and list the
ones from the CallingConv enum. Full descriptions can come later (help
appreciated).

Differential Revision: https://reviews.llvm.org/D151996

llvm/docs/AMDGPUUsage.rst

@@ -1064,6 +1064,67 @@ The AMDGPU backend supports the following LLVM IR attributes.
 
      ======================================= ==========================================================
 
+Calling Conventions
+-------------------
+
+The AMDGPU backend supports the following calling conventions:
+
+  .. table:: AMDGPU Calling Conventions
+     :name: amdgpu-cc
+
+     =============================== ==========================================================
+     Calling Convention              Description
+     =============================== ==========================================================
+     ``ccc``                         The C calling convention. Used by default.
+                                     See :ref:`amdgpu-amdhsa-function-call-convention-non-kernel-functions`
+                                     for more details.
+
+     ``fastcc``                      The fast calling convention. Mostly the same as the ``ccc``.
+
+     ``coldcc``                      The cold calling convention. Mostly the same as the ``ccc`.
+
+     ``amdgpu_cs``                   Used for Mesa/AMDPAL compute shaders.
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_es``                   Used for AMDPAL shader stage before geometry shader if geometry is in
+                                     use. So either the domain (= tessellation evaluation) shader if
+                                     tessellation is in use, or otherwise the vertex shader.
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_gfx``                  Used for AMD graphics targets. Functions with this calling convention
+                                     cannot be used as entry points.
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_gs``                   Used for Mesa/AMDPAL geometry shaders.
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_hs``                   Used for Mesa/AMDPAL hull shaders (= tessellation control shaders).
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_kernel``               See :ref:`_amdgpu-amdhsa-function-call-convention-kernel-functions`
+
+     ``amdgpu_ls``                   Used for AMDPAL vertex shader if tessellation is in use.
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_ps``                   Used for Mesa/AMDPAL pixel shaders.
+                                     ..TODO::
+                                     Describe.
+
+     ``amdgpu_vs``                   Used for Mesa/AMDPAL last shader stage before rasterization (vertex
+                                     shader if tessellation and geometry are not in use, or otherwise
+                                     copy shader if one is needed).
+                                     ..TODO::
+                                     Describe.
+
+     =============================== ==========================================================
+
+
 .. _amdgpu-elf-code-object:
 
 ELF Code Object