[PATCH] Add loop unroll pragma support

[Reid Kleckner]

As far as the clang side goes, this all looks good.

As a separate chagne, we should have documentation for '#pragma clang loop ...' that explains the difference between 'vectorize', 'interleave', and 'unroll'.  This should be in clang/docs/LanguageExtensions.html after '#pragma optnone'.  The loop_hint attribute should point at it as the main documentation.

================
Comment at: lib/Sema/SemaStmtAttr.cpp:126-136
@@ -117,1 +125,13 @@
+  // Accumulated state of enable|disable hints for each hint category.
+  bool EnabledIsSet[3] = {false, false, false};
+  int EnabledValue[3];
+  // Accumulated state of numeric hints for each hint category.
+  bool NumericValueIsSet[3] = {false, false, false};
+  int NumericValue[3];
 
+  int EnableOptionId[3] = {LoopHintAttr::Vectorize, LoopHintAttr::Interleave,
+                           LoopHintAttr::Unroll};
+  int NumericOptionId[3] = {LoopHintAttr::VectorizeWidth,
+                            LoopHintAttr::InterleaveCount,
+                            LoopHintAttr::UnrollCount};
+
----------------
Rather than having 6 parallel arrays, maybe this would be better as an array of a struct:
  struct {
    int EnableOptionId;
    int NumericOptionId;
    bool EnabledIsSet;
    bool ValueIsSet;
    bool Enabled;
    int Value;
  } Options[] = {
    {LoopHintAttr::Vectorize, LoopHintAttr::VectorizeWidth},
    {LoopHintAttr::Interleave, LoopHintAttr::InterleaveCount},
    {LoopHintAttr::Unroll, LoopHintAttr::UnrollCount}
  };

http://reviews.llvm.org/D4089