Revise implementation of case-insensitive command names

Store both the as-written and lower-case command names and use
the latter to avoid case-insensitive string comparisons.

With this I obtain 2-6% speed increase (on Windows) for the configure
step with no significant changes in memory usage.  A case-insensitive
comparison is a lot slower than just calling `==` because the operator
will use things like memcmp, so prefer the latter.

The `cmSystemTools::LowerCase` function allocates a new string each time
it is called, so before this change we were allocating in:

* cmMakefile::Configure two times for each function
  (to look for `cmake_minimum_required` and `project`)
* cmMakefile::ExecuteCommand twice by function by calling
  cmState::GetCommand and copying the name

Now we are only allocating once by function instead of four.

Source/cmListFileCache.h

@@ -23,11 +23,22 @@ class cmMessenger;
 
 struct cmCommandContext
 {
-  std::string Name;
+  struct cmCommandName
+  {
+    std::string Lower;
+    std::string Original;
+    cmCommandName() {}
+    cmCommandName(std::string const& name) { *this = name; }
+    cmCommandName& operator=(std::string const& name);
+  } Name;
   long Line;
   cmCommandContext()
-    : Name()
-    , Line(0)
+    : Line(0)
+  {
+  }
+  cmCommandContext(const char* name, int line)
+    : Name(name)
+    , Line(line)
   {
   }
 };
@@ -81,7 +92,7 @@ public:
     cmListFileContext lfc;
     lfc.FilePath = fileName;
     lfc.Line = lfcc.Line;
-    lfc.Name = lfcc.Name;
+    lfc.Name = lfcc.Name.Original;
     return lfc;
   }
 };