Allow searching for prebuilt implicit modules.

This reverts commit c67656b994c87224e0b33e2c4b09093986a5cfa6, and addresses the
build issue.

Revert "Allow searching for prebuilt implicit modules."

This reverts commit 71e108cd86e70b06c5fa3a63689dcb3555c3d13f.

This change caused a build failure on Windows:
http://lab.llvm.org:8011/#/build

Revert "Allow searching for prebuilt implicit modules."

This reverts commit 71e108cd86e70b06c5fa3a63689dcb3555c3d13f.

This change caused a build failure on Windows:
http://lab.llvm.org:8011/#/builders/83/builds/570

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Allow searching for prebuilt implicit modules.

The behavior is controlled by the `-fprebuilt-implicit-modules` option, and
allows searching for implicit modules in the prebuilt module cache paths.

Allow searching for prebuilt implicit modules.

The behavior is controlled by the `-fprebuilt-implicit-modules` option, and
allows searching for implicit modules in the prebuilt module cache paths.

The current command-line options for prebuilt modules do not allow to easily
maintain and use multiple versions of modules. Both the producer and users of
prebuilt modules are required to know the relationships between compilation
options and module file paths. Using a particular version of a prebuilt module
requires passing a particular option on the command line (e.g.
`-fmodule-file=[<name>=]<file>` or `-fprebuilt-module-path=<directory>`).

However the compiler already knows how to distinguish and automatically locate
implicit modules. Hence this proposal to introduce the
`-fprebuilt-implicit-modules` option. When set, it enables searching for
implicit modules in the prebuilt module paths (specified via
`-fprebuilt-module-path`). To not modify existing behavior, this search takes
place after the standard search for prebuilt modules. If not

Here is a workflow illustrating how both the producer and consumer of prebuilt
modules would need to know what versions of prebuilt modules are available and
where they are located.

clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v1 <config 1 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v2 <config 2 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v3 <config 3 options>

clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules_v1 <config 1 options>
clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap <non-prebuilt config options>

With prebuilt implicit modules, the producer can generate prebuilt modules as
usual, all in the same output directory. The same mechanisms as for implicit
modules take care of incorporating hashes in the path to distinguish between
module versions.

Note that we do not specify the output module filename, so `-o` implicit modules are generated in the cache path `prebuilt_modules`.

clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 1 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 2 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 3 options>

The user can now simply enable prebuilt implicit modules and point to the
prebuilt modules cache. No need to "parse" command-line options to decide
what prebuilt modules (paths) to use.

clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules -fprebuilt-implicit-modules <config 1 options>
clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules -fprebuilt-implicit-modules <non-prebuilt config options>

This is for example particularly useful in a use-case where compilation is
expensive, and the configurations expected to be used are predictable, but not
controlled by the producer of prebuilt modules. Modules for the set of
predictable configurations can be prebuilt, and using them does not require
"parsing" the configuration (command-line options).

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D68997

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