[HLSL] add loop unroll (#93879)

spec: https://github.com/microsoft/hlsl-specs/pull/263

- `Attr.td` - Define the HLSL loop attribute hints (unroll and loop)
- `AttrDocs.td` - Add documentation for unroll and loop
- `CGLoopInfo.cpp` - Add codegen for HLSL unroll that maps to clang
unroll expectations
- `ParseStmt.cpp` - For statements if HLSL define DeclSpecAttrs via
MaybeParseMicrosoftAttributes
- `SemaStmtAttr.cpp` - Add the HLSL loop unroll handeling

resolves #70114

dxc examples: 
- for loop: https://hlsl.godbolt.org/z/8EK6Pa139
- while loop:  https://hlsl.godbolt.org/z/ebr5MvEcK
- do while: https://hlsl.godbolt.org/z/be8cedoTs 

Documentation:

![Screenshot_20240531_143000](https://github.com/llvm/llvm-project/assets/1802579/9da9df9b-68a6-49eb-9d4f-e080aa2eff7f)

1 files changed, 13 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGLoopInfo.cpp b/clang/lib/CodeGen/CGLoopInfo.cpp
index 0d4800b..6b886bd 100644
--- a/clang/lib/CodeGen/CGLoopInfo.cpp
+++ b/clang/lib/CodeGen/CGLoopInfo.cpp
@@ -612,9 +612,9 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
     const LoopHintAttr *LH = dyn_cast<LoopHintAttr>(Attr);
     const OpenCLUnrollHintAttr *OpenCLHint =
         dyn_cast<OpenCLUnrollHintAttr>(Attr);
-
+    const HLSLLoopHintAttr *HLSLLoopHint = dyn_cast<HLSLLoopHintAttr>(Attr);
     // Skip non loop hint attributes
-    if (!LH && !OpenCLHint) {
+    if (!LH && !OpenCLHint && !HLSLLoopHint) {
       continue;
     }
 
@@ -635,6 +635,17 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
         Option = LoopHintAttr::UnrollCount;
         State = LoopHintAttr::Numeric;
       }
+    } else if (HLSLLoopHint) {
+      ValueInt = HLSLLoopHint->getDirective();
+      if (HLSLLoopHint->getSemanticSpelling() ==
+          HLSLLoopHintAttr::Spelling::Microsoft_unroll) {
+        if (ValueInt == 0)
+          State = LoopHintAttr::Enable;
+        if (ValueInt > 0) {
+          Option = LoopHintAttr::UnrollCount;
+          State = LoopHintAttr::Numeric;
+        }
+      }
     } else if (LH) {
       auto *ValueExpr = LH->getValue();
       if (ValueExpr) {