[ClangImporter] Do not construct CodeGenOpts when not needed
In #90037, ClangImporter is updated to fix the CodeGen target info when
using directCC1 mode. This causes some regression on specific platforms
for emitPCM actions. EmitPCM action in explicit module build only get a
minimal set of the swift flags and on some platforms the minimal flags
cannot generate clang driver arguments that can be accepted by
ClangImporter.
The fix is to not regenerating driver arguments for CodeGen target
options when there are no CodeGen is in the RequestedAction. This also
saves the unnecessary round trip into clang driver when CodeGenOption is
not needed.
rdar://181058499
