Introduce distinct error codes for precise capturing

compiler/rustc_error_codes/src/error_codes/E0799.md

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+Something other than a type or const parameter has been used when one was
+expected.
+
+Erroneous code example:
+
+```compile_fail,E0799
+fn bad1() -> impl Sized + use<main> {}
+
+fn bad2(x: ()) -> impl Sized + use<x> {}
+
+fn main() {}
+```
+
+In the given examples, for `bad1`, the name `main` corresponds to a function
+rather than a type or const parameter. In `bad2`, the name `x` corresponds to
+a function argument rather than a type or const parameter.
+
+Only type and const parameters, including `Self`, may be captured by
+`use<...>` precise capturing bounds.

compiler/rustc_error_codes/src/error_codes/E0800.md

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+A type or const parameter of the given name is not in scope.
+
+Erroneous code examples:
+
+```compile_fail,E0800
+fn missing() -> impl Sized + use<T> {}
+```
+
+To fix this error, please verify you didn't misspell the type or const
+parameter, or double-check if you forgot to declare the parameter in
+the list of generics.