[clang] [clang-repl] Fix module contamination after failed template instantiation (PR #204152)

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

The fix: in the error path, call `HandleTranslationUnit` to flush and discard the "partial IR", then call `GenModule` to start a fresh module for the next cell. `HandleTranslationUnit` on an error path nullifies the underlying module pointer, so `GenModule `needs to fall back to the stored `LLVMContext` rather than asking the (`null`) module for its context.

https://github.com/llvm/llvm-project/pull/204152