IWYU: Add more mappings for std::__decay_and_strip

IWYU incorrectly classifies this internal STL type as not internal, and
suggests including `<type_traits>` for it.  Work around the problem by
mapping the offending names to a file that we always include.

See include-what-you-use issue 434.

Utilities/IWYU/mapping.imp

@@ -52,11 +52,13 @@
   # __decay_and_strip is used internally in the C++11 standard library.
   # IWYU does not classify it as internal and suggests to add <type_traits>.
   # To ignore it, we simply map it to a file that is included anyway.
+  # Use '-Xiwyu -v7' to see the fully qualified names that need this.
   # TODO: Can this be simplified with an @-expression?
   #{ symbol: [ "@std::__decay_and_strip<.*>::__type", private, "\"cmConfigure.h\"", public ] },
   { symbol: [ "std::__decay_and_strip<cmCommand *&>::__type", private, "\"cmConfigure.h\"", public ] },
   { symbol: [ "std::__decay_and_strip<cmGeneratorTarget *&>::__type", private, "\"cmConfigure.h\"", public ] },
   { symbol: [ "std::__decay_and_strip<cmFindCommon::PathLabel &>::__type", private, "\"cmConfigure.h\"", public ] },
+  { symbol: [ "std::__decay_and_strip<cmSearchPath>::__type", private, "\"cmConfigure.h\"", public ] },
   { symbol: [ "std::__decay_and_strip<std::basic_string<char> &>::__type", private, "\"cmConfigure.h\"", public ] },
   { symbol: [ "std::__decay_and_strip<const std::basic_string<char> &>::__type", private, "\"cmConfigure.h\"", public ] },
   { symbol: [ "std::__decay_and_strip<cmFindPackageCommand::PathLabel &>::__type", private, "\"cmConfigure.h\"", public ] },