[libclang] In clang_equalCursors, clear out the "FirstInDeclGroup" part in a declaration cursor
before doing the comparison.

This is because we can't set "FirstInDeclGroup" consistently; for example, when visiting a DeclStmt we will set
it but we don't set it on the result of clang_getCursorDefinition for a reference of the same declaration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171878 91177308-0d34-0410-b5e6-96231b3b80d8