Support matching signext attribute in vla-lambda-capturing test to appease clang-cmake-mips builder.

The Mips target adds the signext attribute to signed 32-bit integers in order
to support the N32/N64 correctly. Integers must be promoted to 64-bit bit on
these ABI's.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222617 91177308-0d34-0410-b5e6-96231b3b80d8