[OpenMP] Remove "alternativeName" from Clause and Directive classes

The "alternativeName" was introduced ~5 years ago in D82405, and at
the moment it has only one use, the one that the original change
introduced.

OpenMP 6.0 spec has introduced different spellings of some directives,
but the "alternativeName" is not an adequate mechanism to implement it.

For those reasons remove the "alternativeName" member from both
Clause and Directive in DirectiveBase.td.

Since the alternativeName text appeared in a number of clang OpenMP
tests, implement `getOpenMPClauseNameForDiag` in SemaOpenMP.cpp to
preserve the text of the diagnostics.

Author: kparzysz