[CMake][libc] Don't put archive in build/lib/<target triple> by default

[aidengrossman]

Summary

ea8f4b98419750c8cc7c60ea43b570adf47b3f78 broke some build configurations
because it was enabled by default and some people are using a just built
libc/clang/LLVM to work on other projects where having a just built LLVM
libc in one of Clang's default include directories can make things
unusable.

[goldstein.w.n]

LGTM, but wait on a proper maintainer before pushing.
Thanks!

[michaelrj]

LGTM

[sivachandra]

This is OK but can you point us to what broke?

[aidengrossman]

@goldstein.w.n reported this failure with full build mode after this patch:

$> echo "int main() {}" > tmp.c &&  ./bin/clang tmp.c -o tmp
/bin/ld: /lib/x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x21): undefined reference to `__libc_start_main'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

As far as I understand it, the previous patch also moved the full build libc.a and libm.a into the build/lib/<target triple> directory, so clang would try and link against the full built LLVM libc there by default instead of the system libc, causing build failures. It didn't break the overlay mode because llvmlibc.a needs to be explicitly linked against.

In addition it seems to make more sense to keep this gated behind a flag even just for the overlay mode as LLVM libc matures further.

libc/CMakeLists.txt

 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # The top-level source directory.
 set(LIBC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
-if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
+set(LIBC_ENABLE_USE_BY_CLANG OFF CACHE BOOL "Whether or not to place libc in a build directory findable by a just built clang")
+
+if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND LIBC_ENABLE_USE_BY_CLANG)
   set(LIBC_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
 else()
-  if(LLVM_LIBRARY_OUTPUT_INTDIR)
+  if(NOT LIBC_ENABLE_USE_BY_CLANG)
+    set(LIBC_LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
+  elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
     set(LIBC_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   else()
     set(LIBC_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
   endif()
 endif()
 
 if(LLVM_LIBC_FULL_BUILD OR LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
   if(NOT LIBC_HDRGEN_EXE)