[lldb] Fix gcc 5.4.0 compile error

[DavidSpickett]

Summary

Specify type when constructing PromotionKeys,
this fixes error:
"chosen constructor is explicit in copy-initialization"
when compiling lldb with GCC 5.4.0.

This is due to std::tuple having an explicit
default constructor, see:
http://cplusplus.github.io/LWG/lwg-defects.html#2193

[DavidSpickett]

https://www.llvm.org/docs/GettingStarted.html gives the minimum gcc as 5.1. (I found this cross compiling, the apt package for AArch64 is 5.4.0)

[labath]

Looks good.

lldb/source/Utility/Scalar.cpp

static bool IsSigned(Scalar::Type type) {
   }
   llvm_unreachable("Unhandled type!");
 }
 
 Scalar::PromotionKey Scalar::GetPromoKey() const {
   Category cat = GetCategory(m_type);
   switch (cat) {
   case Category::Void:
-    return {cat, 0, false};
+    return PromotionKey{cat, 0, false};
   case Category::Integral:
-    return {cat, m_integer.getBitWidth(), !IsSigned(m_type)};
+    return PromotionKey{cat, m_integer.getBitWidth(), !IsSigned(m_type)};
   case Category::Float:
     return GetFloatPromoKey(m_float.getSemantics());
   }
   llvm_unreachable("Unhandled category!");
 }
 
 Scalar::PromotionKey Scalar::GetFloatPromoKey(const llvm::fltSemantics &sem) {
   static const llvm::fltSemantics *const order[] = {
       &APFloat::IEEEsingle(), &APFloat::IEEEdouble(),
       &APFloat::x87DoubleExtended()};
   for (const auto &entry : llvm::enumerate(order)) {
     if (entry.value() == &sem)
-      return {Category::Float, entry.index(), false};
+      return PromotionKey{Category::Float, entry.index(), false};
   }
   llvm_unreachable("Unsupported semantics!");
 }
 
 // Promote to max type currently follows the ANSI C rule for type promotion in
 // expressions.
 Scalar::Type Scalar::PromoteToMaxType(Scalar &lhs, Scalar &rhs) {
   const auto &Promote = [](Scalar &a, const Scalar &b) {