Allow shadowing of predeclared types and built-in functions

[teoxoy]

see https://gpuweb.github.io/gpuweb/wgsl/#predeclared

[SparkyPotato]

Shadowing built-in functions should work in gfx-rs/naga#2075.
Types however are all reserved keywords.

[teoxoy]

Shadowing built-in functions should work in gfx-rs/naga#2075.

I looked at gfx-rs/naga#2075 but it doesn't look to me like user-defined types take priority over predeclared types.

Types however are all reserved keywords.

gfx-rs/naga#2130 will unreserve types

We can resolve this in a separate PR and add tests to make sure we got it working.

[andyleiserson]

I think all the predeclared struct types are named with a double-underscore prefix, but if there are any that are not, when this issue is fixed, it might be worth an additional test case in the struct_type_mismatch_* group in wgsl_errors (added by #7444), to test that a value having the predeclared struct type foo is not assignable to a variable with a user-defined struct foo type with the same members. (This issue does not apply to the double-underscore-prefixed types because identifiers with the double underscore prefix are reserved.)