…to recognize 'levitation syntax extensions'

…y syntax.

AST, Sema: Added support for levitation packages.
Clang tests, AST, Parser: Added tests for levitation packages.

    CXXRecordDecl: Added isLevitationPackage
    Sema, Template Instantiator: added draft support for 'global' keyword.
    Sema: added draft implementation for package class instantiation.

…Decl. It still crashes for default constructor after regular package classes instantiated.

…d comments for Levitation-related Sema methods.

TODO: Still have issues with basic HandleLevitationPackageDependency class.

TagDecl::isPackageDependent finally moved into Decl, with name isLevitationPackageDependent, which relies on Decl::LevitationPackageDependent bitfield.
New Decl::LevitationPackageDependent property behaves as follows:
* It can be true only for non-nested declarations of package namespaces.
1. If such declaration was created by parser, then it dependent ('true').
2. If such declaration was created during template instantiation, then it inharits pattern flag value.
3. If such declaration was created during package decl instantiation, then it is 'false'.

'PackageClassInstantiationMode' has been renamed into 'PackageClassInstantiationStage'

Minor fix in Sema::isLevitationGlobal method.

  Added Levitation mode build stage compiler option.
  Two stages are possible:
   * Build AST stage
   * Build Object file stage

 * Roled back ClassTemplateDecl::getTemplatedDecl

  Added support for package namespaces. Package namespace is dependent context.

 * Added fix for case when we have no lookup entry for decls to be removed.

  Fixed initializations of LevitationPackage flag.

  Changed way of marking levitation package classes as package dependent.
  Since now it is done by Recursive AST Visitor by parser before
  handleTranslationUnit call.

  Fixed Sema::isLevitationGlobal method.

  Added use of LevitationBuildStage value.

  Added Levitation Decls shadowing for Build Obj stage.

Decl::LevitationPackageDependent is not stored in PCH.
Instead introduced Sema::LevitationPackageDependentDecls collection, after
we parsed TU, all dependent classes registered there, and then to be stored
in PCH as a separated record.

…lsInstantiator (which is a DeclVisitor)

…ed namespaces.

…for out of scope record definitions'

…SemaLevitation.cpp

… remained to complete 'computeAutoDependency' method.

Added ASTDependenciesProcessor and its satellite classes.
Removed flevitation_mode and flevitation_build_stage.
Instead introduced '-levitation-build-ast' cc program action with its satellite options ('-levitation-deps-output-file=...', '-levitation-sources-root-dir=...', '-levitation-source-file-extension=...').
Also introduced '-levitation-deps-input-file=...' for future non-implemented part.

…ested name specifier for out of scope record definitions'

… is remained to create second PCH Generator which will emit only declaration part.

Some details:
 * Introduced separate "clang/Levitation" directory.
 * From Sema Dependency-related classes have been moved into clang/Levitation/Dependencies
 * LevitationFrontendActions and LevitationASTConsumers also moved into this dir.
 * Some frontend options have been corrected.

Current syntax for launching BuildAST is:

<clang> -cc1 -xc++ -std=c++17 \
-levitation-build-ast \
-levitation-sources-root-dir=<sources root dir> \
-levitation-deps-output-file=<dependencies output file> \
-levitation-source-file-extension=<levitation source ext with '.' symbol, e.g. '.cppl'> \
<input source files, e.g. 'test-project/P1/A.cppl'>

-levitation-decl-ast-file-extension=<additional extension suffix for decl ast, with '.'> \
[-o <output definition ast file name, also decl ast will be emitted but as well>]

…AST creator. Still draft, .decl-ast files are not created as expected (only tmp copy exists, but fail rename doesn't happen).

…t' implementation. We should handle properly .ast and .decl-ast input files.

…. We should rework it though.

…rge. This commit reverts previous one.

…evitationAST input kind.

…itationBuildObjectAction

…to ParserPostProcessor AST consumer. Also some minor refactoring was made.

…erBuilder.

…ould reuse ASTReader and call ReadASTCore several times during its lifetime, since it looks like this use-case is supported and used during chained modules read (through IMPORTS PCH block).

…dependency and main file. But with two very draft FIXMEs though. Should be reworked in next commit: probably we could introduce ModuleKind::MK_LevitationDependency which is almost clone of MK_PCH, with minor difference (see commit changes).)

…d unused code.