[OpenACC][NFC] Fix GCC7 build after fec003a

A GCC 7 requirement on the completeness of types before using them in a
variant means we need a constructor for these (or define them outside the
current thing).  This patch should fix the build, as reported by
@Kewen12

Author: erichkeane

clang/include/clang/Sema/SemaOpenACC.h

@@ -135,6 +135,11 @@ class SemaOpenACC : public SemaBase {
     struct RangeForInfo {
       const CXXForRangeStmt *Uninstantiated = nullptr;
       const CXXForRangeStmt *CurrentVersion = nullptr;
+      // GCC 7.x requires this constructor, else the construction of variant
+      // doesn't work correctly.
+      RangeForInfo() : Uninstantiated{nullptr}, CurrentVersion{nullptr} {}
+      RangeForInfo(const CXXForRangeStmt *Uninst, const CXXForRangeStmt *Cur)
+          : Uninstantiated{Uninst}, CurrentVersion{Cur} {}
     };
 
     struct ForInfo {