[OpenMP] Fix runtimes default build (#149871)

The default build of openmp (`cmake -S <llvm-project>/runtimes
-DLLVM_ENABLE_RUNTIMES=openmp`) current fails with
```
CMake Error at /home/meinersbur/src/llvm/flangrt/_src/cmake/Modules/GetClangResourceDir.cmake:17 (string):
  string sub-command REGEX, mode MATCH needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  /home/meinersbur/src/llvm/flangrt/_src/openmp/CMakeLists.txt:126 (get_clang_resource_dir)
```
The reason is that because it is not a bootstrapping-build, the clang
resource dir that it intends to write files such as `omp-tools.h` into,
is unavailable. Using the Clang resource dir for writing files is
conceptually broken, as that dir might be located in
`/usr/lib/clang/<version>/`. Writing to it is only intended in
bootstrapping builds where Clang is built alongside openmp.

This patch unifies the identification of being in a bootstrapping built.
The same `LLVM_TREE_AVAILABLE` definition is going to be used in
#137828. No reason for each runtime to define its own.

openmp/CMakeLists.txt

@@ -115,7 +115,7 @@ option(OPENMP_ENABLE_LIBOMPTARGET "Enable building libomptarget for offloading."
 option(OPENMP_ENABLE_LIBOMP_PROFILING "Enable time profiling for libomp." OFF)
 
 # Header install location
-if(${OPENMP_STANDALONE_BUILD})
+if(NOT LLVM_TREE_AVAILABLE)
   set(LIBOMP_HEADERS_INSTALL_PATH "${CMAKE_INSTALL_INCLUDEDIR}")
 else()
   include(GetClangResourceDir)

openmp/runtime/src/CMakeLists.txt

@@ -11,7 +11,7 @@
 include(ExtendPath)
 
 # The generated headers will be placed in clang's resource directory if present.
-if(OPENMP_STANDALONE_BUILD OR NOT LLVM_RUNTIMES_BUILD)
+if(NOT LLVM_TREE_AVAILABLE)
   set(LIBOMP_HEADERS_INTDIR ${CMAKE_CURRENT_BINARY_DIR})
 else()
   set(LIBOMP_HEADERS_INTDIR ${LLVM_BINARY_DIR}/${LIBOMP_HEADERS_INSTALL_PATH})

runtimes/CMakeLists.txt

@@ -92,6 +92,14 @@ include(LLVMCheckCompilerLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
+
+# Determine whether we are in the runtimes/runtimes-bins directory of a
+# bootstrap build.
+set(LLVM_TREE_AVAILABLE OFF)
+if (LLVM_LIBRARY_OUTPUT_INTDIR AND LLVM_RUNTIME_OUTPUT_INTDIR AND PACKAGE_VERSION)
+  set(LLVM_TREE_AVAILABLE ON)
+endif()
+
 # CMake omits default compiler include paths, but in runtimes build, we use
 # -nostdinc and -nostdinc++ and control include paths manually so this behavior
 # is undesirable. Filtering CMAKE_{LANG}_IMPLICIT_INCLUDE_DIRECTORIES to remove