Speedup ClangToLLVMArgMapping construction. NFC.

Add a method to calculate the number of arguments given QualType
expnads to. Use this method in ClangToLLVMArgMapping calculation.
This number may be cached in CodeGenTypes for efficiency, if needed.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218623 91177308-0d34-0410-b5e6-96231b3b80d8