Move compile_commands to the gen folder

[ryuwd]

Moved compile_commands.json to gen/ and made sure gen/ is created before executing build targets. Tested on mu2ebuild01 with fresh clone of Offline

N.B.

• compile_commands.json is created for any build
• scons -Q compiledb generates a compile_commands.json without executing other build targets
• compile_commands.json is removed correctly after scons -c

[FNALbuild]

Hi @ryuwd,
You have proposed changes to files in these packages:

• /
• scripts

which require these tests: build.

FNALbuild is explained here.

[kutschke]

@FNALbuild run build test

[FNALbuild]

⌛ The following tests have been triggered for ref ced1d50: build

[FNALbuild]

☀️
The build test passed at ref ced1d50. Total build time: 735.909241 seconds.

For more details, please check here.