[Parser] Emit descriptive diagnostic for misplaced pragma

[Parser] Emit descriptive diagnostic for misplaced pragma

If a class or struct or union declaration contains a pragma that
is not valid in this context, compiler issues generic error like
"expected member name or ';' after declaration specifiers". With this
change the error tells that this pragma cannot appear in this declaration.

Differential Revision: https://reviews.llvm.org/D64932

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367779 91177308-0d34-0410-b5e6-96231b3b80d8