[llvm-config] Remove --src-root option

[mgorny]

Summary

Remove the --src-root option from the deprecated llvm-config tool.
None of the llvm-project projects use this option anymore. The value
was only meaningful for in-tree use and usually became no longer correct
once LLVM was installed -- either because it was built in a temporary
directory, or installed from a binary package and built on a different
system entirely. Therefore, third-party tools could not have been
relying on it anyway.

The LLVM_SRC_ROOT #define is left intact, as it is used to compute
includedir when llvm-config is used in-source.

[tstellar]

LGTM.

[phosek]

LGTM

llvm/docs/CommandGuide/llvm-config.rst

 **--prefix**
 
  Print the installation prefix for LLVM.
 
 **--shared-mode**
 
  Print how the provided components can be collectively linked (`shared` or `static`).
 
-**--src-root**
-
- Print the source root from which LLVM was built.
-
 **--system-libs**
 
  Print all the system libraries needed to link against the specified LLVM
  *components*, including any dependencies.
 
 **--targets-built**
 
  Print the component names for all targets supported by this copy of LLVM.

llvm/tools/llvm-config/llvm-config.cpp

Options:\n\
   --libfiles        Fully qualified library filenames for makefile depends.\n\
   --libnames        Bare library names for in-tree builds.\n\
   --libs            Libraries needed to link against LLVM components.\n\
   --link-shared     Link the components as shared libraries.\n\
   --link-static     Link the component libraries statically.\n\
   --obj-root        Print the object root used to build LLVM.\n\
   --prefix          Print the installation prefix.\n\
   --shared-mode     Print how the provided components can be collectively linked (`shared` or `static`).\n\
-  --src-root        Print the source root LLVM was built from.\n\
   --system-libs     System Libraries needed to link against LLVM components.\n\
   --targets-built   List of all targets currently built.\n\
   --version         Print LLVM version.\n\
 Typical components:\n\
   all               All LLVM libraries (default).\n\
   engine            Either a native JIT or a bitcode interpreter.\n";
   if (ExitWithFailure)
     exit(1);
#endif
       } else if (Arg == "--build-system") {
         OS << LLVM_BUILD_SYSTEM << '\n';
       } else if (Arg == "--has-rtti") {
         OS << (LLVM_HAS_RTTI ? "YES" : "NO") << '\n';
       } else if (Arg == "--shared-mode") {
         PrintSharedMode = true;
       } else if (Arg == "--obj-root") {
         OS << ActivePrefix << '\n';
-      } else if (Arg == "--src-root") {
-        OS << LLVM_SRC_ROOT << '\n';
       } else if (Arg == "--ignore-libllvm") {
         LinkDyLib = false;
         LinkMode = BuiltSharedLibs ? LinkModeShared : LinkModeAuto;
       } else if (Arg == "--link-shared") {
         LinkMode = LinkModeShared;
       } else if (Arg == "--link-static") {
         LinkMode = LinkModeStatic;
       } else if (Arg == "--help") {