[RISCV][VLS] Support RISCV VLS calling convention (#100346)

This patch adds a function attribute `riscv_vls_cc` for RISCV VLS
calling
convention which takes 0 or 1 argument, the argument is the `ABI_VLEN`
which is the `VLEN` for passing the fixed-vector arguments, it wraps the
argument as a scalable vector(VLA) using the `ABI_VLEN` and uses the
corresponding mechanism to handle it. The range of `ABI_VLEN` is [32,
65536],
if not specified, the default value is 128.

Here is an example of VLS argument passing:
Non-VLS call:
```
  void original_call(__attribute__((vector_size(16))) int arg) {}
=>
  define void @original_call(i128 noundef %arg) {
  entry:
    ...
    ret void
  }
```
VLS call:
```
  void __attribute__((riscv_vls_cc(256))) vls_call(__attribute__((vector_size(16))) int arg) {}
=>
  define riscv_vls_cc void @vls_call(<vscale x 1 x i32> %arg) {
  entry:
    ...
    ret void
  }
}
```

The first Non-VLS call passes generic vector argument of 16 bytes by
flattened integer.
On the contrary, the VLS call uses `ABI_VLEN=256` which wraps the
vector to <vscale x 1 x i32> where the number of scalable vector
elements
is calaulated by: `ORIG_ELTS * RVV_BITS_PER_BLOCK / ABI_VLEN`.
Note: ORIG_ELTS = Vector Size / Type Size = 128 / 32 = 4.

PsABI PR: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/418
C-API PR: https://github.com/riscv-non-isa/riscv-c-api-doc/pull/68

1 files changed, 12 insertions, 0 deletions

diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
index 3a511de553ad..c50410dc365b 100644
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -3061,6 +3061,18 @@ enum CXCallingConv {
   CXCallingConv_M68kRTD = 19,
   CXCallingConv_PreserveNone = 20,
   CXCallingConv_RISCVVectorCall = 21,
+  CXCallingConv_RISCVVLSCall_32 = 22,
+  CXCallingConv_RISCVVLSCall_64 = 23,
+  CXCallingConv_RISCVVLSCall_128 = 24,
+  CXCallingConv_RISCVVLSCall_256 = 25,
+  CXCallingConv_RISCVVLSCall_512 = 26,
+  CXCallingConv_RISCVVLSCall_1024 = 27,
+  CXCallingConv_RISCVVLSCall_2048 = 28,
+  CXCallingConv_RISCVVLSCall_4096 = 29,
+  CXCallingConv_RISCVVLSCall_8192 = 30,
+  CXCallingConv_RISCVVLSCall_16384 = 31,
+  CXCallingConv_RISCVVLSCall_32768 = 32,
+  CXCallingConv_RISCVVLSCall_65536 = 33,
 
   CXCallingConv_Invalid = 100,
   CXCallingConv_Unexposed = 200