[clang][deps] Remove dependency on llvm targets from lib DependencyScanning

In d64eccf we split the object file reader from the writer and removed
the dependency on clangCodeGen from the dependency scanner. However, we
were still initializing all the llvm targets, which was needed for
writing object file pcms but not reading them. This caused us to link
more llvm target code than necessary. Shrinks clangd by nearly 50% for
me.

rdar://144790713

Author: benlangmuir

clang/lib/Tooling/DependencyScanning/CMakeLists.txt

@@ -1,5 +1,4 @@
 set(LLVM_LINK_COMPONENTS
-  ${LLVM_TARGETS_TO_BUILD}
   Core
   Option
   Support

clang/lib/Tooling/DependencyScanning/DependencyScanningService.cpp

@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
-#include "llvm/Support/TargetSelect.h"
 
 using namespace clang;
 using namespace tooling;
@@ -17,10 +16,4 @@ DependencyScanningService::DependencyScanningService(
     ScanningMode Mode, ScanningOutputFormat Format,
     ScanningOptimizations OptimizeArgs, bool EagerLoadModules, bool TraceVFS)
     : Mode(Mode), Format(Format), OptimizeArgs(OptimizeArgs),
-      EagerLoadModules(EagerLoadModules), TraceVFS(TraceVFS) {
-  // Initialize targets for object file support.
-  llvm::InitializeAllTargets();
-  llvm::InitializeAllTargetMCs();
-  llvm::InitializeAllAsmPrinters();
-  llvm::InitializeAllAsmParsers();
-}
+      EagerLoadModules(EagerLoadModules), TraceVFS(TraceVFS) {}