cmMakefile: return a backtrace

This allows backtraces to be fully controlled by the makefile rather
than externally (and makes changing how they are manipulated easier).
cmake: remove dummy backtraces for IssueMessage
genex: remove the need for backtraces

Rather than making dummy backtraces and passing them around, just make
backtraces optional.
backtrace: Convert to local paths in IssueMessage

This is the only place we care show the FilePath to the user, so defer
the expensive relative path calculation until here.
revert old backtrace-performance changes
Merge topic 'revert/old-backtrace-performance' into next

88962807 revert old backtrace-performance changes
Merge topic 'dev/backtrace-performance' into next

f73569d0 backtrace: Convert to local paths in IssueMessage
01061c55 genex: remove the need for backtraces
1a617e82 cmake: remove dummy backtraces for IssueMessage
a08c03b6 cmMakefile: return a backtrace