fileapi: Compute codemodel compile groups without target-wide settings

Previously we computed the entire description of each source file including all target-wide settings, and then computed compile groups using those complete descriptions. This is inefficient when target-wide settings are large because they are included in comparisons even though they are the same for every source. Instead compute source groups using only the source-specific settings, and then merge the target-wide settings into place only once per unique compile group.

This is a slight behavior change in the case that a source-specific compile definition duplicates a target-wide definition. Previously that source would still be grouped with other sources that do not have that definition. Now that source will be in its own compile group even though it ultimately compiles with the same settings as another group. This is acceptable because the source is specified by the project with source-specific settings already.

Fixes: #19520 (closed)