Rename 'DarwinStaticLib' to 'DarwinLibName'

The former name doesn't make sense, we are using this parameter for both .a and .dylib libraries.

No functional change.

http://reviews.llvm.org/D6040



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220939 91177308-0d34-0410-b5e6-96231b3b80d8