Revert "build-lldb-mi: Disallow detecting LLVM outside of $LLVM_DIR"

This reverts commit 9e628b0f2d.

If LLVM was built with --disable-dylib, then the individual
LLVM libraries may expose a dependency on zlib. By restricting
the lldb-mi build to detect dependencies outside of $LLVM_DIR,
the build of lldb-mi fails to find zlib, making it impossible to
find all the necessary dependencies, ending up with errors like this:

    CMake Error at llvm-mingw/llvm-project/llvm/build-asserts/lib/cmake/llvm/LLVMExports.cmake:55 (set_target_properties):
      The link interface of target "LLVMSupport" contains:

        ZLIB::ZLIB

      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.

Revert this change for now, to allow building lldb-mi against
a non-dylib build of LLVM.

build-lldb-mi.sh

@@ -65,7 +65,7 @@ else
     esac
 fi
 
-LLVM_DIR="$PREFIX"
+export LLVM_DIR="$PREFIX"
 
 # Try to find/guess the builddir under the llvm buildtree next by.
 # If LLVM was built without LLVM_INSTALL_TOOLCHAIN_ONLY, and the LLVM
@@ -91,7 +91,7 @@ if [ -d "$LLVM_SRC" ]; then
     done
     if [ -n "$DIRS" ]; then
         dir="$(ls -td $DIRS | head -1)"
-        LLVM_DIR="$LLVM_SRC/$dir"
+        export LLVM_DIR="$LLVM_SRC/$dir"
         echo Using $LLVM_DIR as LLVM build dir
         break
     else
@@ -121,13 +121,13 @@ if [ -n "$HOST" ]; then
         exit 1
         ;;
     esac
-fi
 
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH=$LLVM_DIR"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY"
+fi
 
 if [ -n "$MACOS_REDIST" ]; then
     : ${MACOS_REDIST_ARCHS:=arm64 x86_64}