[llvm] [cmake] Include httplib in LLVMConfig.cmake (#79305)

Include LLVM_ENABLE_HTTPLIB along with httplib package finding in
LLVMConfig.cmake, as this dependency is needed by LLVMDebuginfod that is
now used by LLDB. Without it, building LLDB standalone fails with:

```
CMake Error at /usr/lib/llvm/19/lib64/cmake/llvm/LLVMExports.cmake:90 (set_target_properties):
  The link interface of target "LLVMDebuginfod" contains:

    httplib::httplib

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  /usr/lib/llvm/19/lib64/cmake/llvm/LLVMConfig.cmake:357 (include)
  cmake/modules/LLDBStandalone.cmake:9 (find_package)
  CMakeLists.txt:34 (include)
```

(cherry picked from commit 3c9f34c)

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -90,6 +90,11 @@ if(LLVM_ENABLE_CURL)
   find_package(CURL)
 endif()
 
+set(LLVM_ENABLE_HTTPLIB @LLVM_ENABLE_HTTPLIB@)
+if(LLVM_ENABLE_HTTPLIB)
+  find_package(httplib)
+endif()
+
 set(LLVM_WITH_Z3 @LLVM_WITH_Z3@)
 
 set(LLVM_ENABLE_DIA_SDK @LLVM_ENABLE_DIA_SDK@)