use relative lines in completion tests

[sam-mccall]

tests under clang/test/CodeComplete are hard to maintain because they contain -code-complete-at=%s:123:45 everywhere, and inserting/deleting text requires rewriting all the line numbers.
Probably other clang tests have this problem, but these are the worst.

Lit supports %(line) and FileCheck supports [[@LINE]] (and also {{.*}} where appropriate) to deal with this. %(line) was added in 2012 and never used - I wonder whether this is related to Google's downstream testrunner not supporting it :-)

plan:

• add %(line) support to the private downstream testrunner
• add a script to apply these replacements based on regex search
• apply the script to the CodeComplete tests

[llvmbot]

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