[clang][CGStmt] fix crash on invalid asm statement

Clang is crashing on the following statement

  char var[9];
  __asm__ ("" : "=r" (var) : "0" (var));

This is similar to existing test: crbug_999160_regtest

The issue happens when EmitAsmStmt is trying to convert input to match
output type length. However, that is not guaranteed to be successful all the
time and if the statement itself is invalid like having an array type in
the example, we should give a regular error message here instead of
using assert().

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D120596