Support MS builtins using 'long' on LP64 platforms

This allows for -fms-extensions to work the same on LP64. For example,
_BitScanReverse is expected to be 32-bit, matching Windows/LLP64, even
though long is 64-bit on x86_64 Darwin or Linux (LP64).

Implement this by adding a new character code 'N', which is 'int' if
the target is LP64 and the same 'L' otherwise

Differential Revision: https://reviews.llvm.org/D34377

rdar://problem/32599746

llvm-svn: 305875