[clang] Improve clarity of the implicit declaration diagnostic (PR #149314)

[Corentin Jabot via cfe-commits]

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@@ -816,11 +816,12 @@ def warn_unreachable_association : Warning<
 
 /// Built-in functions.
 def ext_implicit_lib_function_decl : ExtWarn<
-  "implicitly declaring library function '%0' with type %1">,
-  InGroup<ImplicitFunctionDeclare>;
+  "implicitly declaring library function '%0' with%select{| standards-mandated}2 "
+  "type %1">, InGroup<ImplicitFunctionDeclare>;
 def ext_implicit_lib_function_decl_c99 : ExtWarn<
-  "call to undeclared library function '%0' with type %1; ISO C99 and later "
-  "do not support implicit function declarations">,
+  "call to undeclared library function '%0', will assume it exists with"
+  "%select{| standards-mandated}2 type %1; ISO C99 and later do not support "
+  "implicit function declarations">,
   InGroup<ImplicitFunctionDeclare>, DefaultError;
----------------
cor3ntin wrote:

A way to simplify (ie, get rid of `isDeclaredInStandardHeader`)
would be to say " will assume it exists with type %1 as if header <%2>" had been included"

(where %2 is ` Context.BuiltinInfo.getName(ID)`)

WDYT?


https://github.com/llvm/llvm-project/pull/149314