Add language standard aliases for -std=c18, -std=gnu18, and -std=iso9899:2018.

Add language standard aliases for -std=c18, -std=gnu18, and -std=iso9899:2018.

As described in D40225, the C17 standard was balloted and approved in 2017, but the ISO publication process delayed the actual publication until 2018. WG14 considers the release to be C17 and describes it as such, but users can still be confused by the publication year which is why -std=c18 adds value. These aliases map to c17 and are all supported by GCC 8.x with the same behavior. Note that the value of __STDC_VERSION__ remains at 201710L.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@344749 91177308-0d34-0410-b5e6-96231b3b80d8