This patch fixes the following error:
```
llvm/lib/Support/TextEncoding.cpp:274:11: error: cannot initialize a variable of type 'char *' with an rvalue of type 'const char *'
274 | char *Input = InputLength ? const_cast<char *>(Source.data()) : "";
| ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
These are identified by misc-include-cleaner. I've filtered out those
that break builds. Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.