[flang][nfc] Update README.md

[awarzynski]

Summary

With https://reviews.llvm.org/D116731 merged, installing Clang, MLIR or
LLVM is no longer required for standalone builds. For consistency sake,
remove "installation" from the build instrucitons.

[PeteSteinfeld]

flang/README.md

39–46

This section states that it's necessary to create the install area for a later standalone build to succeed. But this is no longer needed.

[awarzynski]

flang/README.md

39–46

Thanks for pointing this out!

[awarzynski]

Update the "Building flang" section, as per Pete's suggestion.

[PeteSteinfeld]

Looks great!

Thanks for doing this, Andrzej. You rock.

[awarzynski]

In D117100#3238183, @PeteSteinfeld wrote:

Thanks for doing this, Andrzej. You rock.

Thank you, Pete, that's very kind of you! Users like you make it worthwhile :)

flang/README.md

 If you're interested in contributing to the compiler,
 read the [style guide](docs/C++style.md)
 and
 also review [how flang uses modern C++ features](docs/C++17.md).
 
 If you are interested in writing new documentation, follow 
 [markdown style guide from LLVM](https://github.com/llvm/llvm-project/blob/main/llvm/docs/MarkdownQuickstartTemplate.md).
 
 ## Building flang
-There are two ways to build flang.  The first method is to build it at the same
-time that you build all of the projects on which it depends.  This is called
-building in tree.  The second method is to first do an in tree build to create
-all of the projects on which flang depends, then build an install area for
-these projects, and then only build the flang code itself.  This is called
-building standalone.  Building standalone has the advantage of being smaller
-and faster.  Once you create the base build and base install areas, you can
-create multiple standalone builds using them.  
+There are two ways to build flang. The first method is to build it at the same
+time that you build all of the projects on which it depends. This is called
+building in tree. The second method is to first do an in tree build to create
+all of the projects on which flang depends, and then only build the flang code
+itself. This is called building standalone. Building standalone has the
+advantage of being smaller and faster. Once you create the base build and base
+install areas, you can create multiple standalone builds using them.
 
 Note that instructions for building LLVM can be found at
 https://llvm.org/docs/GettingStarted.html.
 
 ### Building flang in tree
 Building flang in tree means building flang along with all of the projects on
 which it depends.  These projects include mlir, clang, flang, and compiler-rt.
 Note that compiler-rt is only needed to access libraries that support 16 bit
 floating point numbers.  It's not needed to run the automated tests.
 
 Here's a complete set of commands to clone all of the necessary source and do
 the build.  
 
 First clone the source:
 ```bash
 git clone https://github.com/llvm/llvm-project.git my-project
 ```
 Once the clone is complete, execute the following commands:
 ```bash
 cd my-project
-INSTALLDIR=`pwd`/install
 
 rm -rf build
-rm -rf install
 mkdir -p build
 
 cd build
 
 cmake \
   -G Ninja \
   ../llvm \
   -DCMAKE_BUILD_TYPE=Release \
   -DFLANG_ENABLE_WERROR=On \
   -DLLVM_ENABLE_ASSERTIONS=ON \
   -DLLVM_TARGETS_TO_BUILD=host \
   -DCMAKE_INSTALL_PREFIX=$INSTALLDIR
   -DLLVM_LIT_ARGS=-v \
   -DLLVM_ENABLE_PROJECTS="clang;mlir;flang;compiler-rt"
 
 ninja
 ```
 
 To run the flang tests on this build, execute the command in the "build"
 directory:
 ```bash
 ninja check-flang
 ```
 
-If you're happy with the results, the next step is to create the install area.
-While in the `build` directory, run the command:
-```bash
-ninja install
-```
-
 Note that these instructions specify flang as one of the projects to build in
 the in tree build.  This is not strictly necessary for subsequent standalone
 builds, but doing so lets you run the flang tests to verify that the source
 code is in good shape.
 ### Building flang standalone
 To do the standalone build, start by building flang in tree as described above.
 This build is base build for subsequent standalone builds.  Start each
 standalone build the same way by cloning the source for llvm-project:
cmake \
   -G Ninja \
   -DCMAKE_BUILD_TYPE=Release \
   -DFLANG_ENABLE_WERROR=On \
   -DLLVM_TARGETS_TO_BUILD=host \
   -DLLVM_ENABLE_ASSERTIONS=On \
   -DLLVM_BUILD_MAIN_SRC_DIR=$base/build/lib/cmake/llvm \
   -DLLVM_LIT_ARGS=-v \
   -DLLVM_DIR=$base/build/lib/cmake/llvm \
-  -DCLANG_DIR=$base/install/lib/cmake/clang \
-  -DMLIR_DIR=$base/install/lib/cmake/mlir \
+  -DCLANG_DIR=$base/build/lib/cmake/clang \
+  -DMLIR_DIR=$base/build/lib/cmake/mlir \
   ..
 
 ninja
 ```
-Note that for Clang and MLIR you use the installation directory ($base/install)
-and for LLVM you use the build directory (`$base/build`).   This is not a typo
-in the script.  Rather, it is because running the tests requires the GTest
-infrastructure which is only available in the LLVM build area.  The build also
-requires the `AddClang.cmake` script from Clang, which is only available in the
-install area.
 
 To run the flang tests on this build, execute the command in the "flang/build"
 directory:
 ```bash
 ninja check-flang
 ```
 
 ## Supported C++ compilers