[OpenMP] Remove "alternativeName" from Clause and Directive classes (llvm#138179)

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