Add a document describing in detail how to use clang's "-ftime-trace"
option, and the ClangBuildAnalyzer tool, to find the source files,
header files and templates which slow down Scylla's build the most.
I've used this tool in the past to reduce Scylla build time - see
commits:
fa7a302130 (reduced 6.5%)
f84094320d (reduced 0.1%)
6ebf32f4d7 (reduced 1%)
d01e1a774b (reduced 4%)
I'm hoping that documenting how to use this tool will allow other
developers to suggest similar commits.
Refs #1.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Closes#15209
