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.cxx

@@ -13,6 +13,14 @@
 #include <assert.h>
 #include <sstream>
 
+cmCommandContext::cmCommandName& cmCommandContext::cmCommandName::operator=(
+  std::string const& name)
+{
+  this->Original = name;
+  this->Lower = cmSystemTools::LowerCase(name);
+  return *this;
+}
+
 struct cmListFileParser
 {
   cmListFileParser(cmListFile* lf, cmListFileBacktrace const& lfbt,