Summary:
When the ClangModulesDeclVendor currently fails it just prints very basic and often incomplete diagnostics without any source locations:
```
(lldb) p @import Foundation
error: while importing modules:
'foo/bar.h' file not found
could not build module 'Darwin'
[...]
```
or even just
```
(lldb) p @import Foundation
error: while importing modules:
could not build module 'Darwin'
[...]
```
These diagnostics help neither the user nor us with figuring out what is the reason for the failure.
This patch wires up a full TextDiagnosticPrinter in the ClangModulesDeclVendor and makes
sure we always return the error stream to the user when we fail to compile our modules.
Fixes rdar://63216849
Reviewers: aprantl, jdoerfert
Reviewed By: aprantl
Subscribers: JDevlieghere
Differential Revision: https://reviews.llvm.org/D79947
Raphael Isemann