[clang] Remove outdated INSTALL.txt

[stephanlachnit]

Summary

Information in this file is outdated, and instead the information from clang/README.txt should be taken instead.

[cor3ntin]

@stephanlachnit do you need me to land this on your behalf? What email should we use?

[stephanlachnit]

In D156890#4649226, @cor3ntin wrote:

@stephanlachnit do you need me to land this on your behalf? What email should we use?

Yes, please! You can use [e-mail]

clang/INSTALL.txt

-//===----------------------------------------------------------------------===//
-// Clang Installation Instructions
-//===----------------------------------------------------------------------===//
-
-These instructions describe how to build and install Clang.
-
-//===----------------------------------------------------------------------===//
-// Step 1: Organization
-//===----------------------------------------------------------------------===//
-
-Clang is designed to be built as part of an LLVM build. Assuming that the LLVM
-source code is located at $LLVM_SRC_ROOT, then the clang source code should be
-installed as:
-
-  $LLVM_SRC_ROOT/tools/clang
-
-The directory is not required to be called clang, but doing so will allow the
-LLVM build system to automatically recognize it and build it along with LLVM.
-
-//===----------------------------------------------------------------------===//
-// Step 2: Configure and Build LLVM
-//===----------------------------------------------------------------------===//
-
-Configure and build your copy of LLVM (see $LLVM_SRC_ROOT/GettingStarted.html
-for more information).
-
-Assuming you installed clang at $LLVM_SRC_ROOT/tools/clang then Clang will
-automatically be built with LLVM. Otherwise, run 'make' in the Clang source
-directory to build Clang.
-
-//===----------------------------------------------------------------------===//
-// Step 3: (Optional) Verify Your Build
-//===----------------------------------------------------------------------===//
-
-It is a good idea to run the Clang tests to make sure your build works
-correctly. From inside the Clang build directory, run 'make test' to run the
-tests.
-
-//===----------------------------------------------------------------------===//
-// Step 4: Install Clang
-//===----------------------------------------------------------------------===//
-
-From inside the Clang build directory, run 'make install' to install the Clang
-compiler and header files into the prefix directory selected when LLVM was
-configured.
-
-The Clang compiler is available as 'clang' and 'clang++'. It supports a gcc like
-command line interface. See the man page for clang for more information.