[inlineasm] Fix an incorrect warning about register constraint and modifier.

Previously, when the following piece of code was compiled, clang would
incorrectly warn that the size of "wide_two" does not match register size
specified by the constraint and modifier":

long wide_two = two;
asm ("%w0 %1" : "+r" (one), "+r"(wide_two));

This was caused by a miscalculation of ConstraintIdx in Sema::ActOnGCCAsmStmt.

This commit fixes PR21270 and rdar://problem/18668354.

llvm-svn: 228089