[lldb] Add omitted abstract formal parameters in DWARF symbol files

This patch fixes a problem introduced by clang change
https://reviews.llvm.org/D95617 and described by
https://bugs.llvm.org/show_bug.cgi?id=50076#c6, where inlined functions
omit unused parameters both in the stack trace and in `frame var`
command. With this patch, the parameters are listed correctly in the
stack trace and in `frame var` command.

Specifically, we parse formal parameters from the abstract version of
inlined functions and use those formal parameters if they are missing
from the concrete version.

Differential Revision: https://reviews.llvm.org/D110571