[Clang][NFC] Fix out-of-bounds access (#77193)
The changes to tablegen made by
https://github.com/llvm/llvm-project/pull/76825 result in
`StmtClass::lastStmtConstant` changing from `StmtClass::WhileStmtClass`
to `StmtClass::GCCAsmStmtClass`. Since `CFG::BuildOptions::alwaysAdd` is
never called with a `WhileStmt`, this has flown under the radar until
now.
Once such test in which an out-of-bounds access occurs is
`test/Sema/inline-asm-validate.c`, among many others.
