[clang] `FunctionDecl::getReturnTypeSourceRange()` returns invalid `SourceRange` for trailing return type functions

[bricknerb]

This probably effects a lot of code that uses trailing return type syntax.

To make it easily reproducible, I looked at this use case:

llvm-project/clang/test/SemaCXX/err_typecheck_assign_const.cpp

Line 32 in 4f80c06

const int &return_const_ref(); // expected-note{{function 'return_const_ref' which returns const-qualified type 'const int &' declared here}}

With trailing return type it generates the note without a location.

Not using trailing return types (conventional prefix syntax):

const int& good();
void test_good() { good() = 10; }

Diagnoses with

<source>:2:27: error: cannot assign to return value because function 'good' returns a const value
    2 | void test_good() { good() = 10; }
      |                    ~~~~~~ ^
<source>:1:7: note: function 'good' which returns const-qualified type 'const int &' declared here
    1 | const int& good();
      |       ^~~~
1 error generated.
Compiler returned: 1

However, using trailing return types:

auto bad() -> const int&;
void test_bad() { bad() = 10; }

Diagnoses with

<source>:2:25: error: cannot assign to return value because function 'bad' returns a const value
    2 | void test_bad() { bad() = 10; }
      |                   ~~~~~ ^
note: function 'bad' which returns const-qualified type 'const int &' declared here
1 error generated.
Compiler returned: 1

See https://godbolt.org/z/nn4KTYceW

[AaronBallman]

We don't track the source location of the trailing return type as part of the FunctionTypeLoc; we only track it when parsing the declarator, but it gets lost when we form the type information.