llvm: Simplify plain icon

[why]
While the plain icon is plainer than the original icon, it is still
quite complex with very thin strokes.

[how]
Just take the outermost outlines of the plain icon and crate a solid
icon from it.

Then hand-remove 'dent' in the tail.

Signed-off-by: Fini Jastrow <[email protected]>