SwiftCompilerSources: rework bridging

Introduce two modes of bridging:
* inline mode: this is basically how it worked so far. Using full C++ interop which allows bridging functions to be inlined.
* pure mode: bridging functions are not inlined but compiled in a cpp file. This allows to reduce the C++ interop requirements to a minimum. No std/llvm/swift headers are imported.

This change requires a major refactoring of bridging sources. The implementation of bridging functions go to two separate files: SILBridgingImpl.h and OptimizerBridgingImpl.h.
Depending on the mode, those files are either included in the corresponding header files (inline mode), or included in the c++ file (pure mode).

The mode can be selected with the BRIDGING_MODE cmake variable. By default it is set to the inline mode (= existing behavior). The pure mode is only selected in certain configurations to work around C++ interop issues:
* In debug builds, to workaround a problem with LLDB's `po` command (rdar://115770255).
* On windows to workaround a build problem.

include/module.modulemap

@@ -1,7 +1,5 @@
 module BasicBridging {
-  header "swift/Basic/BridgedSwiftObject.h"
   header "swift/Basic/BasicBridging.h"
-  header "swift/Basic/SourceLoc.h"
   requires cplusplus
   export *
 }
@@ -11,17 +9,7 @@ module CBasicBridging {
 }
 
 module ASTBridging {
-  header "swift/AST/AnyFunctionRef.h"
   header "swift/AST/ASTBridging.h"
-  header "swift/AST/Builtins.h"
-  header "swift/AST/DiagnosticEngine.h"
-  header "swift/AST/DiagnosticConsumer.h"
-  header "swift/AST/ForeignAsyncConvention.h"
-  header "swift/AST/ForeignErrorConvention.h"
-  header "swift/AST/SubstitutionMap.h"
-
-  textual header "swift/AST/Builtins.def"
-
   requires cplusplus
   export *
 }
@@ -32,7 +20,6 @@ module CASTBridging {
 
 module SILBridging {
   header "swift/SIL/SILBridging.h"
-  header "swift/SIL/SILLocation.h"
   requires cplusplus
   export *
 }