[clang] Thread Safety Analysis: Very basic capability alias-analysis (PR #142955)

[Marco Elver via cfe-commits]

melver wrote:

Some more self-review, and fixes:

- Rename createVariable() -> translateVarDecl() for clarify.
- Reintroduce createThisPlaceholder() for clarify.
- Handle escaping aliases through pass by-non-const-ref (or pointer), and add more tests.

https://github.com/llvm/llvm-project/pull/142955