add implementation-internal namespace for globalasm

[davidtwco]

Fixes #138261

Adds a namespace for global_asm with a lowercase letter which is reserved for implementation-internal disambiguation:

Lowercase letters are reserved for implementation-internal disambiguation categories (and demanglers should never show them)

As a implementation-internal disambiguation category, the demangler implementations shouldn't need updated (i.e. if this were an uppercase letter, then our mangle-then-demangle checks would fail because the demangler would expect to have explicit handling). 'a' is chosen arbitrarily, for asm, but I can change it to something else if preferred.

#[rustc_symbol_name] only looks at top-level items, and would need a bunch of changes to be able to check the symbol for foo::{constant}::{closure} in the global_asm in this test, so for now the test just checks this compiles.

The alternative to this would be to prohibit declaration of items in the operand of a global_asm, which is a breaking change.

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[Kivooeo]

Looks reasonable, I also thinks that 'a' is fine

r=me when ci is green

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