Driver: out-of-line static analyzer flag handling (NFC)

Extract the analyzer flag handling into its own function to reduce the
overall complexity of the construction of the clang compiler arguments.
NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312124 91177308-0d34-0410-b5e6-96231b3b80d8