[cherry-pick swift/release/6.0][clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds

[ian-twilightcoder]

Once a file has been #import'ed, it gets stamped as if it was #pragma once and will not be re-entered, even on #include. This means that any errant #import of a file designed to be included multiple times, such as <assert.h>, will incorrectly mark it as include-once and break the multiple include functionality. Normally this isn't a big problem, e.g. <assert.h> can't have its NDEBUG mode changed after the first #import, but it is still mostly functional. However, when clang modules are involved, this can cause the header to be hidden entirely.

Objective-C code most often uses #import for everything, because it's required for most Objective-C headers to prevent double inclusion and redeclaration errors. (It's rare for Objective-C headers to use macro guards or #pragma once.) The problem arises when a submodule includes a multiple-include header. The "already included" state is global across all modules (which is necessary so that non-modular headers don't get compiled into multiple translation units and cause redeclaration errors). If another module or the main file #import's the same header, it becomes invisible from then on. If the original submodule is not imported, the include of the header will effectively do nothing and the header will be invisible. The only way to actually get the header's declarations is to somehow figure out which submodule consumed the header, and import that instead. That's basically impossible since it depends on exactly which modules were built in which order.

#import is a poor indicator of whether a header is actually include-once, as the #import is external to the header it applies to, and requires that all inclusions correctly and consistently use #import vs #include. When modules are enabled, consider a header marked textual in its module as a stronger indicator of multiple-include than #import's indication of include-once. This will allow headers like <assert.h> to always be included when modules are enabled, even if #import is erroneously used somewhere.

rdar://123790578
(cherry picked from commit fd02ed8)

[ian-twilightcoder]

Explanation: This change allows #import to re-enter textual headers when modules are enabled, which is necessary for upcoming SDK changes.
Radar (and possibly SR Issue): rdar://123790578
Scope: The scope should be fairly small as it only applies to textual headers in module maps, which are not common.
Risk: Inherent risk in changing the behavior of #import.
Testing: Unit tests, world builds.
Reviewed By: Jan Svoboda (jansvoboda11)

[ian-twilightcoder]

@swift-ci test llvm

[ian-twilightcoder]

@swift-ci test