[runtimes] Calculate the path to the llvm directory without the LLVM cmake package

[mstorsjo]

Summary

If building by pointing cmake directly at the llvm-project/runtimes
directory, the llvm cmake package files (that provide e.g.
LLVM_BUILD_MAIN_SRC_DIR) aren't necessarily available. Instead just
use a path relative to the current source dir.

[phosek]

LGTM

runtimes/CMakeLists.txt

 endif()
 
 # Setting these variables will allow the sub-build to put their outputs into
 # the library and bin directories of the top-level build.
 set(LLVM_LIBRARY_OUTPUT_INTDIR ${LLVM_LIBRARY_DIR})
 set(LLVM_RUNTIME_OUTPUT_INTDIR ${LLVM_TOOLS_BINARY_DIR})
 
 # This variable makes sure that e.g. llvm-lit is found.
-set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR})
+set(LLVM_MAIN_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 set(LLVM_CMAKE_PATH ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 
 # This variable is used by individual runtimes to locate LLVM files.
-set(LLVM_PATH ${LLVM_BUILD_MAIN_SRC_DIR})
+set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 
 if(APPLE)
   set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 endif()
 
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)