Support `func.call` / Remove single-function assumption

[AlexanderViand-Intel]

Currently, various places in the compiler assume that the entire program is contained inside a single function. However, we clearly want to support the ability to call other functions, though probably with the limitation that the call graph must be trivially resolvable at compile time and must have no cycles (i.e., no recursion).

Related PRs:

• #1309
• #1310

[github-actions[bot]]

This issue has 1 outstanding TODOs:

• lib/Analysis/LayoutFoldingAnalysis/LayoutFoldingAnalysis.cpp:32: if the anchor is an arg of a non-main func, it may not be

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