[lldb][CMake] Infer Clang_DIR if not passed explicitly

Summary:
If we only get `LLVM_DIR` and find Clang in the same provided build-tree, automatically infer `Clang_DIR` like this:

```
LLVM_DIR = /path/to/build-llvm/lib/cmake/llvm
Clang_DIR = /paht/to/build-llvm/lib/cmake/clang
```

Reviewers: JDevlieghere, jingham, xiaobai, compnerd, labath

Reviewed By: JDevlieghere, labath

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65798

llvm-svn: 372210

Author: weliveindetail

lldb/cmake/modules/LLDBStandalone.cmake

@@ -1,7 +1,7 @@
 option(LLVM_INSTALL_TOOLCHAIN_ONLY "Only include toolchain files in the 'install' target." OFF)
 
-find_package(LLVM REQUIRED CONFIG HINTS "${LLVM_DIR}" NO_CMAKE_FIND_ROOT_PATH)
-find_package(Clang REQUIRED CONFIG HINTS "${Clang_DIR}" NO_CMAKE_FIND_ROOT_PATH)
+find_package(LLVM REQUIRED CONFIG HINTS ${LLVM_DIR} NO_CMAKE_FIND_ROOT_PATH)
+find_package(Clang REQUIRED CONFIG HINTS ${Clang_DIR} ${LLVM_DIR}/../clang NO_CMAKE_FIND_ROOT_PATH)
 
 # We set LLVM_CMAKE_PATH so that GetSVN.cmake is found correctly when building SVNVersion.inc
 set(LLVM_CMAKE_PATH ${LLVM_CMAKE_DIR} CACHE PATH "Path to LLVM CMake modules")

lldb/docs/resources/build.rst

@@ -120,10 +120,11 @@ Standalone builds
 *****************
 
 This is another way to build LLDB. We can use the same source-tree as we
-checked out above, but now we will have two build-trees:
+checked out above, but now we will have multiple build-trees:
 
 * the main build-tree for LLDB in ``/path/to/lldb-build``
-* a provided build-tree for LLVM and Clang in ``/path/to/llvm-build``
+* one or more provided build-trees for LLVM and Clang; for simplicity we use a
+  single one in ``/path/to/llvm-build``
 
 Run CMake with ``-B`` pointing to a new directory for the provided
 build-tree\ :sup:`1` and the positional argument pointing to the ``llvm``
@@ -139,15 +140,15 @@ Clang. Then we build the ``ALL`` target with ninja:
 
 Now run CMake a second time with ``-B`` pointing to a new directory for the
 main build-tree and the positional argument pointing to the ``lldb`` directory
-in the source-tree. In order to find the provided build-tree, the build-system
-needs the options ``LLVM_DIR`` and ``Clang_DIR`` (CMake variables are
-case-sensitive!):
+in the source-tree. In order to find the provided build-tree, the build system
+looks for the path to its CMake modules in ``LLVM_DIR``. If you use a separate
+build directory for Clang, remember to pass its module path via ``Clang_DIR``
+(CMake variables are case-sensitive!):
 
 ::
 
   > cmake -B /path/to/lldb-build -G Ninja \
           -DLLVM_DIR=/path/to/llvm-build/lib/cmake/llvm \
-          -DClang_DIR=/path/to/llvm-build/lib/cmake/clang \
           [<more cmake options>] /path/to/llvm-project/lldb
   > ninja lldb
 
@@ -320,7 +321,6 @@ Build LLDB standalone for development with Xcode:
   > cmake -B /path/to/lldb-build \
           -C /path/to/llvm-project/lldb/cmake/caches/Apple-lldb-Xcode.cmake \
           -DLLVM_DIR=/path/to/llvm-build/lib/cmake/llvm \
-          -DClang_DIR=/path/to/llvm-build/lib/cmake/clang \
           llvm-project/lldb
   > open lldb.xcodeproj
   > cmake --build /path/to/lldb-build --target check-lldb