[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:

Same as above.
Also, the standard names it "ordinary literal encoding", maybe we should follow it

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