fix elast configuration on Windows targets

A typo and missing header inclusion was obscured by the litany of user
defined literal warnings.  This fixes the detection of ELAST on windows.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290941 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed