Set the minimum possible version to 0 instead of 1

The default value of the Version parameter in get<Lang><Enum>Name()
was 0, which was not in the range [1, 0x7fffffff]. To fix this, it
was either to change the default value to 1, or to lower the minimum
version to 0. The latter seemed like a better choice, since 0 is a
natural choice for a lower bound on version numbers.

Author: kparzysz

llvm/include/llvm/Frontend/Directive/DirectiveBase.td

@@ -52,15 +52,15 @@ class DirectiveLanguage {
 }
 
 // Base class for versioned entities.
-class Versioned<int min = 1, int max = 0x7FFFFFFF> {
+class Versioned<int min = 0, int max = 0x7FFFFFFF> {
   // Mininum version number where this object is valid.
   int minVersion = min;
 
   // Maximum version number where this object is valid.
   int maxVersion = max;
 }
 
-class Spelling<string s, int min = 1, int max = 0x7FFFFFFF>
+class Spelling<string s, int min = 0, int max = 0x7FFFFFFF>
     : Versioned<min, max> {
   string spelling = s;
 }

llvm/include/llvm/Frontend/Directive/Spelling.h

@@ -17,7 +17,9 @@ namespace llvm::directive {
 
 struct VersionRange {
   static constexpr int MaxValue = std::numeric_limits<int>::max();
-  int Min = 1;
+  // The default "Version" value in get<Lang><Enum>Name() is 0, include that
+  // in the maximum range.
+  int Min = 0;
   int Max = MaxValue;
 };