[clang] Include the error message in file reading error diagnostic

[hans]

Summary

in order to provide as much information as possible to the user. The diagnostic will now look like for example:

error: error reading '/tmp/foo.c': Permission denied

(This addresses a FIXME from 2019, 9ef6c49baf45)

[thakis]

Lg

[thakis]

(if the presubmit is related, maybe some flang test needs updating? Weird that flang uses clang's diags.)

[hans]

In D146280#4201769, @thakis wrote:

(if the presubmit is related, maybe some flang test needs updating? Weird that flang uses clang's diags.)

The presubmit errors look unrelated and the flang tests seem fine locally.