[clang] [llvm] Enable fexec-charset option (PR #138895)

[Sergei Barannikov via cfe-commits]

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@@ -9401,7 +9318,9 @@ def _SLASH_source_charset : CLCompileJoined<"source-charset:">,
   HelpText<"Set source encoding, supports only UTF-8">,
   Alias<finput_charset_EQ>;
 def _SLASH_execution_charset : CLCompileJoined<"execution-charset:">,
-  HelpText<"Set runtime encoding, supports only UTF-8">,
+  HelpText<"Set the execution <charset> for string and character literals. "
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s-barannikov wrote:

I'm looking at N3220, `6.2.9 Encodings`:
```
The literal encoding is an implementation-defined mapping of the characters of the execution character
set to the values in a character constant (6.4.4.5) or string literal (6.4.5). It shall support a mapping
from all the basic execution character set values into the implementation-defined encoding. It may
contain multibyte character sequences (5.2.2).
```
It doesn't mention "ordinary" (C++ standard does), but it is still encoding, not character set.


https://github.com/llvm/llvm-project/pull/138895