[cmake] fix a typo in llvm_config macro

Summary:
The macro parses out the USE_SHARED option out of the argument list, but
then ignores it and accesses the variable with the same name instead. It
seems the intention here was to check the argument value.

Technically, this is NFC, because the only in-tree usage
(add_llvm_executable) of USE_SHARED sets both the variable and the
argument when calling llvm_config, but it makes the usage of this macro
for out-of-tree users more sensible.

Reviewers: mgorny, beanz

Reviewed By: mgorny

Subscribers: foutrelis, llvm-commits

Differential Revision: https://reviews.llvm.org/D44420

llvm-svn: 334082