[clang] [clang] Fix handling of asm string literals (PR #169803)

[Abhina Sree via cfe-commits]

abhina-sree wrote:

> > As to why we want to upstream this, any users of the upstream compiler would need this change to work on z/OS.
> 
> Any user, really? As best I can tell, this is only for some very specific situations, not for inline assembly in general? (I'm not familiar with the code patterns you're having to deal with; if this is needed by ~100% of folks, that's very different from if it's needed by ~1% of folks.)

Full inline assembly support on z/OS depends on this change. We also plan to upstream [#pragma map](https://www.ibm.com/docs/en/open-xl-c-cpp-zos/2.2.0?topic=sp-pragma-map) after this, which is functionally equivalent to asm labels and is used extensively in the z/OS system headers. Both asm strings and #pragma map symbol names frequently contain EBCDIC escape sequences to handle variant characters across EBCDIC code pages, so both must follow the same string parsing and encoding rules.

https://github.com/llvm/llvm-project/pull/169803