[FileCheck] Improve printing variables with escapes (#145865)

Firstly fix FileCheck printing string variables
double-escaped (first regex, then C-style).
    
This is confusing because it is not clear if the printed
value is the literal value or exactly how it is escaped, without
looking at FileCheck's source code.
    
Secondly, only escape when doing so makes it easier to read the value
(when the string contains tabs, newlines or non-printable characters).
When the variable value is escaped, make a note of it in the output too,
in order to avoid confusion.
    
The common case that is motivating this change is variables that contain
windows style paths with backslashes. These were printed as
`"C:\\\\Program Files\\\\MyApp\\\\file.txt"`.
Now prefer to print them as `"C:\Program Files\MyApp\file.txt"`.
Printing the value literally also makes it easier to search for
variables in the output, since the user can just copy-paste it.